using System;

using System.Collections.Generic;

​

public class Program

{

    public static IEnumerable<string> Get(string input)

    {

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

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

            {

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

                start = i;

            }

    }

​

    public static void Main()

    {

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

        Console.WriteLine(result);

    }

}