static void Main(string[] args)
        {
            StudentGrades stu = new StudentGrades();

            stu.InputSemesters();
            WriteLine();
            stu.EnterGrades();
            stu.CumGpa();
            WriteLine();
            WriteLine("{0}", stu);
            WriteLine();
            SingleSemesterGpa(stu);
            WriteLine();
            WriteLine("{0}", stu);
            WriteLine();
            WriteLine("Total GPA is:  {0:F2}.", stu.TotalGpa);
        }
        public static void SingleSemesterGpa(StudentGrades stu)
        {
            string ans    = "";
            double things = 0;

            do
            {
                Write("For what semester would you like a GPA for?  ");
                int  num;
                bool useNum = int.TryParse(ReadLine(), out num);

                while (num < 1 || num > 4)
                {
                    WriteLine("Number needs to be between 1 and {0}", stu.Semesters);
                    Write("For what semester would you like a GPA for?  ");
                    useNum = int.TryParse(ReadLine(), out num);
                }
                ;
                things = stu.SemesterGpa(num);
                WriteLine("The GPA for semester {0} is {1:F2}", num, things);
                WriteLine("Would you like another semester GPA?  ");
                ans = ReadLine();
            } while (ans.ToLower() == "yes");
        }