using System;

using System.Collections.Generic;

using System.Linq;

​

class Program

{

    static void Main()

    {

        var value = "esyyy hgrrk";

        Console.WriteLine($"Исходный: {value}");

        Console.WriteLine($"Результат: {Format(value)}");

    }

​

    static string Format(string value)

    {

        var result = new List<char>();

​

        int counter = 0;

        foreach (var chr in value)

        {

            if (result.LastOrDefault() == chr)

            {

                counter++;

            }

            else

            {

                if (counter != 0)

                {

                    result.AddRange($"{counter}");

                    counter = 0;

                }

​

                result.Add(chr);

            }

        }

​

        return string.Join(null, result);

    }

}