using System;

public class Program

{

    public static void Main()

    {

        string str ="Hello World";

        int lastIndex = str.Length - 1;

        char[] chars = str.ToCharArray();

        for(int i=0;i<lastIndex;i++,lastIndex--)

        {

            var temp = chars[i];

            chars[i] = chars[lastIndex];

            chars[lastIndex]=temp;

        }

        str = string.Concat(chars);

        Console.WriteLine(str);

    }

}