static void Main(string[] args)
{
var courseService = new CourseService();
Console.Write("How many students for course A? ");
var quantity = int.Parse(Console.ReadLine());
var A = courseService.AddStudents(quantity);
Console.Write("How many students for course B? ");
quantity = int.Parse(Console.ReadLine());
var B = courseService.AddStudents(quantity);
Console.Write("How many students for course C? ");
quantity = int.Parse(Console.ReadLine());
var C = courseService.AddStudents(quantity);
var D = new SortedSet <int>();
D.UnionWith(A);
D.UnionWith(B);
D.UnionWith(C);
Console.Clear();
courseService.Print("A", A);
courseService.Print("B", B);
courseService.Print("C", C);
Console.WriteLine();
Console.WriteLine("Total Students: " + D.Count);
}