using System;

​

public class Run

{

    public static int IndexOfLongestRun(string str)

    {

        var longestRunStart = -1;

        var longestRunLength = 0;

        for (var i = 0; i < str.Length;)

        {

            var runValue = str[i];

            var runStart = i;

            while (++i < str.Length && str[i] == runValue);

            var runLength = i - runStart;

            if (longestRunLength < runLength)

            {

                longestRunStart = runStart;

                longestRunLength = runLength;

            }

        }

        return longestRunStart;

    }

​

    public static void Main(string[] args)

    {

        Console.WriteLine(IndexOfLongestRun("abbcccddddcccbba"));

    }

}