public static void Main()
        {
            Student pesho = new Student("Pesho", "Peshev", "213400","032555555","[email protected]", 1);
            Student ivan = new Student("Ivan", "Ivanov", "213406","03255555","[email protected]", 2);
            Student kina = new Student("Kina", "Kinova", "213806","029999999","[email protected]", 1);
            Student pina = new Student("Pina", "Pinova", "213490","0266666","[email protected]", 2);

            pesho.Marks.Add(2);
            pesho.Marks.Add(2);
            ivan.Marks.Add(6);
            kina.Marks.Add(6);

            List<Student> students= new List<Student>();
            students.Add(pesho);
            students.Add(ivan);
            students.Add(kina);
            students.Add(pina);

            //Problem 9. Student groups

            Console.WriteLine();
            Console.WriteLine("Students from group 2 (Linq) ");
            var studentGroup =
                from student in students
                where student.GroupNumber == 2
                orderby student.FirstName
                select student;

            foreach (var student in studentGroup)
            {
                Console.WriteLine(student.FirstName);
            }

            Console.WriteLine("Students from group 2 (Extension m): ");

               //Problem 10. Student groups extensions
               //Implement the previous using the same query expressed with extension methods.

              var studentsGroup2 = students.SelectByGroup();
              foreach (var student in studentsGroup2)
              {
              Console.WriteLine(student.FirstName);
              }

            Console.WriteLine();

            /*  Problem 11. Extract students by email
                Extract all students that have email in abv.bg.
                Use string methods and LINQ.
             */

            var studentsEmailAbv =
                from student in students
                where student.Email.Contains("@abv.bg")
                select student;
            foreach (var item in studentsEmailAbv)
            {
                Console.WriteLine(item.FirstName);
            }

            /* Problem 12. Extract students by phone

                Extract all students with phones in Sofia.
                Use LINQ.
             * */

            var studentsPhoneSofia =
                from student in students
                where student.PhoneNumber.StartsWith("02") || student.PhoneNumber.StartsWith("+3592")
                select student;

            //Problem 13. Extract students by marks
               //Select all students that have at least one mark Excellent (6) into a new anonymous class that has properties – FullName and Marks.
               //Use LINQ.

            var studentsExcellent =
                from student in students
                where student.Marks.Contains(6)
                select new { FullName = student.FirstName + " " + student.LastName, Marks = student.Marks };

            foreach (var student in studentsExcellent)
            {
                Console.WriteLine(student.FullName);
            }

            //Problem 14. Extract students with two marks
            //Write down a similar program that extracts the students with exactly two marks "2".
            //Use extension methods.

            var studentsGroup3 = students.ExtractTwoMarks();
            Console.WriteLine("Students with exactly two marks 2:");
            foreach (var student in studentsGroup2)
            {
                Console.WriteLine(student.FirstName);
            }

            Console.WriteLine();

               //Problem 15. Extract marks
               //Extract all Marks of the students that enrolled in 2006. (The students from 2006 have 06 as their 5-th and 6-th digit in the FN).

            var students2006 =
                from student in students
                where student.FN.Substring(4,2)=="06"
                select student.Marks;

            foreach (var item in students2006)
            {
                foreach (var mark in item)
                {
                    Console.WriteLine(mark);
                }

            }

              //Problem 17. Longest string
              //Write a program to return the string with maximum length from an array of strings.
              //Use LINQ.

            string[] stringsArray = { "bla", "blabla", "blablabla" };
            string max = stringsArray.OrderByDescending(s => s.Length).First();
            Console.Write("Longest string: ");
            Console.WriteLine(max);

               //Problem 18. Grouped by GroupNumber
               //Create a program that extracts all students grouped by GroupNumber and then prints them to the console.
               //Use LINQ.

               Console.WriteLine(" With Linq");
            Console.WriteLine();
            var result = students.OrderBy(st => st.GroupNumber);
            foreach (var student in result)
            {
                Console.WriteLine("{0}, Group: {1}",student.FirstName,student.GroupNumber);
            }
            Console.WriteLine();
            Console.WriteLine("With extension");
            Console.WriteLine();
            //Using Extensions
            var resultWithExtensions = students.OrderByGroupNumber();
            foreach (var resultWithExtension in resultWithExtensions)
            {
                Console.WriteLine("{0}, Group: {1}", resultWithExtension.FirstName, resultWithExtension.GroupNumber);
            }
        }
        static void Main()
        {
            var students = new List<Student>()
            {
                new Student("Petar","Petrov",23,234106, 3,"02/9738987","[email protected]", new List<byte>{4,4,6,5,2} ),
                new Student("Aleks","Petrov",23,234104, 2,"02/9738987","[email protected]", new List<byte>{2,4,5,5,2} ),
                new Student("Viktoria","Zdravkova",33,660202, 3,"02/8538987","[email protected]", new List<byte>{6,6,6,5,2}),
                new Student("Georgy","Aleksiev",23,110206, 1,"037/6738987","[email protected]", new List<byte>{5,5,6,5,2}),
                new Student("Antonia","Malinova",14,552504, 2,"029/735987","[email protected]", new List<byte>{4,2,3,5,2}),
                new Student("Zdravko","Cvetkov",67,204102, 2,"029/708987","[email protected]", new List<byte>{4,2,6,5,2}),
                new Student("Aleks","Ivanov",45,524501, 2,"02/9758987","[email protected]", new List<byte>{4,6,6,5,2} )
            };

            //Task 9:Select only the students that are from group number 2.
            Console.WriteLine("Select only the students that are from group number 2.");
            Console.WriteLine("--------------------------------------------------------------------------.");
            var group2 = from student in students
                         where student.GroupNumber == 2
                         orderby student.FirstName
                         select student;
            foreach(Student student in group2.ToList())
            {
                Console.WriteLine(student.ToString());
                Console.WriteLine();
            }
            //Task 10: Implement the previous using the same query expressed with extension methods.
            Console.WriteLine("Implement the previous using the same query expressed with extension methods.");
            Console.WriteLine("--------------------------------------------------------------------------.");
            var group2wMethods = students.Where(stud => stud.GroupNumber == 2).OrderBy(stud => stud.FirstName);
            foreach (Student student in group2wMethods.ToList())
            {
                Console.WriteLine(student.ToString());
                Console.WriteLine();
            }

            //Task 11: Extract all students that have email in abv.bg
            Console.WriteLine("Extract all students that have email in abv.bg");
            Console.WriteLine("--------------------------------------------------------------------------.");

            var studentInAbv = from student in students
                               where student.Email.Substring(student.Email.IndexOf("@") + 1) == "abv.bg"
                               select student;
            foreach (Student student in studentInAbv.ToList())
            {
                Console.WriteLine(student.ToString());
                Console.WriteLine();
            }

            //Task 12: Extract students by phone
            Console.WriteLine("Extract students by phone");
            Console.WriteLine("--------------------------------------------------------------------------.");
            var studentsInSofia = from student in students
                                  where student.Tel.Substring(0, student.Tel.IndexOf("/")) == "02"
                                  select student;

            foreach (Student student in studentsInSofia.ToList())
            {
                Console.WriteLine(student.ToString());
                Console.WriteLine();
            }

            //Task 13: TODO

            //Task 14: TOFO

            //Task 15: Extract all Marks of the students that enrolled in 2006. (The students from 2006 have 06 as their 5-th and 6-th digit in the FN).
            Console.WriteLine("Extract all Marks of the students that enrolled in 2006. (The students from 2006 have 06 as their 5-th and 6-th digit in the FN).");
            Console.WriteLine("--------------------------------------------------------------------------.");

            var studentWith2Fs = students.Where(st => st.Grades.Count(grade => grade == 2) == 2);
            foreach (Student student in studentWith2Fs.ToList())
            {
                Console.WriteLine(student.ToString());
                Console.WriteLine();
            }

            //Taks 15: Extract all Marks of the students that enrolled in 2006. (The students from 2006 have 06 as their 5-th and 6-th digit in the FN).
            Console.WriteLine("Extract all Marks of the students that enrolled in 2006. (The students from 2006 have 06 as their 5-th and 6-th digit in the FN).");
            Console.WriteLine("--------------------------------------------------------------------------.");

            var marksFrom06 = students.Where(st => st.FN.ToString().Substring(4, 2) == "06").Select(st=> st.Grades).SelectMany(grade=> grade) ;
            foreach (byte grade in marksFrom06.ToList())
            {
                Console.Write("{0} ",grade);
            }
            Console.WriteLine();

            //Task 16 Groups
            Console.WriteLine("Extract all students in Mathematics department");
            Console.WriteLine("--------------------------------------------------------------------------.");
            List<Group> groups = new List<Group>
            {
                new Group(1,"Mathematics"),
                new Group(2,"Biology"),
                new Group(3,"Philosophy"),
                new Group(4,"Literature"),
                new Group(5,"Economy")
            };

            var studentsInMath = from student in students
                                join gr in groups on student.GroupNumber equals gr.GroupID
                                where gr.Department == "Mathematics"
                                select student;

            foreach (Student student in studentsInMath.ToList())
            {
                Console.WriteLine(student.ToString());
                Console.WriteLine();
            }

            //Task 18:Create a program that extracts all students grouped by GroupNumber and then prints them to the console
            Console.WriteLine("Create a program that extracts all students grouped by GroupNumber and then prints them to the console");
            Console.WriteLine("--------------------------------------------------------------------------.");

            var groupedByGroup = from student in students
                                 group student by student.GroupNumber into grouped
                                 select new { groupID = grouped.Key, st = grouped.ToList() };

            foreach (var group in groupedByGroup)
            {
                Console.WriteLine("Students in group {0}:", group.groupID);
                Console.WriteLine("***" );
                foreach(var student in group.st )
                {
                    Console.WriteLine("{0} ",student);
                }
                Console.WriteLine();
            }

            //Task 19 Rewrite the previous using extension methods.
            Console.WriteLine("Rewrite the previous using extension methods.");
            Console.WriteLine("---------------------------------------------");
            Dictionary<int, List<Student>> result = students.OrderByGroupNumber();
        }