using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var ch = 'x';

        var n = 3;

        var re = new Regex("((?:[^" +ch+ "]*" +ch+ "){" + (n-1) + "}[^" +ch+ "]*)" +ch);

        var result = re.Replace("I amx nexver axt hoxme on Sxundxaxs", m => m.Groups[1] + "y");

        Console.WriteLine(result);

    }

}