using System;

public class Program

{

    public static void Main()

    {

        var x = "test";

        lock (x) {

            x = f(x);   

        }

        Console.WriteLine(x);

    }

    public static string f(string x) {

        var arr = x.ToCharArray();

        Array.Reverse(arr);

        return new string(arr); 

    }

}