using System;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        string line = "aaabzzzaafff";

        string newString = GetString(line);

        Console.WriteLine(newString);

    }

​

    private static string GetString(string targetLine)

    {

        string targetLineAppended = targetLine + ".";

        StringBuilder filledLineBuilder = new StringBuilder();

        int occurence = 0;

        char previousSymbol = targetLineAppended[0];

        foreach (char symbol in targetLineAppended)

        {

            if (previousSymbol != symbol)

            {

                filledLineBuilder.Append(occurence);

                filledLineBuilder.Append(previousSymbol);

                occurence = 0;

            }

​

            previousSymbol = symbol;

            occurence++;

        }

​

        return filledLineBuilder.ToString();

    }

}