using System;

using System.Collections.Generic;

using System.Text;

​

public class Program

{

    public static IEnumerable<string> Get(string input)

    {

        int i, start;

        for (i = 0, start = 0; i < input.Length; i++)

            if (input[i] != input[start])

            {

                yield return $"{i - start}{input[start]}";

                start = i;

            }

​

        yield return $"{i - start}{input[start]}";

    }

​

    public static string Get2(string input)

    {

        var sb = new StringBuilder();

        int i, start;

        for (i = 0, start = 0; i < input.Length; i++)

            if (input[i] != input[start])

            {

                sb.Append($"{i - start}{input[start]}");

                start = i;

            }

​

        sb.Append($"{i - start}{input[start]}");

        return sb.ToString();

    }

​

    public static void Main()

    {

        var result = string.Concat(Get("xaaabbccaadddd"));

        Console.WriteLine(result);

        Console.WriteLine( Get2("xaaabbccaadddd"););

    }

}