using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(reverseString("Hello World!"));

    }

    public static string reverseString(string str){

        var carray = str.ToCharArray();

        for(var i =0;i<carray.Length/2;i++){

            var ii = (carray.Length-1) - i;

            var old = carray[i];

            var neww = carray[ii];

            carray[ii] = old;

            carray[i] = neww;

        }

        return new String(carray);

    }

}