using System;

public class Program

{

    public static string Reverse(string a)

{

if (a != null && a == string.Empty)

    return string.Empty;

string b = string.Empty;

​

for(int i = a.Length - 1; i>=0;i--)

{

    b = b+a[i];

}

​

return b;

}

    public static void Main()

    {

        Console.WriteLine(Reverse("Hello World"));

    }

}