using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<string> cities = new List<string>();

        cities.Add("New York");

        cities.Add("Mumbai");

        cities.Add("Berlin");

        cities.Add("Istanbul");

​

        string line = string.Join(":", cities.ToArray());

        Console.WriteLine(line);

    }

}