Courses*

using System;
using System.Collections.Generic;
using System.Linq;

namespace Courses
{
    class Program
    {
        static void Main(string[] args)
        {
            var registrationDictionary = new Dictionary<string, List<string>>();
            string command = string.Empty;
            while ((command = Console.ReadLine()) != "end")
            {
                string[] commandArray = command.Split(" : ");
                string course = commandArray[0];
                string student = commandArray[1];
                if (registrationDictionary.ContainsKey(course) == false)
                {
                    registrationDictionary[course] = new List<string>();
                    registrationDictionary[course].Add(student);
                }

                else
                {
                    registrationDictionary[course].Add(student);
                }
            }

            foreach (var kvp in registrationDictionary.OrderByDescending(x=>x.Value.Count()))
            {
                string course = kvp.Key;
                var listOfStudents = kvp.Value;
                Console.WriteLine($"{course}: {listOfStudents.Count()}");
                foreach (var students in listOfStudents.OrderBy(x=>x))
                {
                    Console.WriteLine($"-- {students}");
                }
            }
        }
      }
    }