using System;

public class Program

{

    public static void Main()

    {

        string str = "4eeksforGeeks";

        Console.WriteLine(LeftMostRepeatingChar(str));

    }

    public static int LeftMostRepeatingChar(string str)

    {

        int[] arr = new int[256];

        int res = -1;

        for(var i=str.Length-1; i >= 0; i--)

        {

            arr[str[i]]++;

            if(arr[str[i]] > 1)

            {

                res = i;    

            }

        }

        //Console.WriteLine(string.Join(", ", arr));

        return res;

    }

}