using System;

public class Program

{

    public static void Main()

    {

        var s1 = "123456";

        var indexFound = -1;

        //what is the index of X ?

        //NOte: in JS for the range and caret select the starting index is 1 (not zero)

        //case1: we not remove a char but just add one?

        //s2.lengh > s2.lengh

        var s2 = "123X456";

        var i=0;

        foreach (var s in s1)

        {

            if (s == s2[i] )

            {

                //Console.WriteLine((i + " OK"));

            }else{

                //Console.WriteLine((i + " BAD"));

                indexFound = i;

                break;

            }

            i++;

        }

        Console.WriteLine( (indexFound==-1) ? "notfound": "found::" + indexFound);

    }

}