using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string str = "abcd";    

        Console.WriteLine(Reverse(str));

    }

    public static string Reverse(string str)

    {

        if(str.Length == 0) return "";

        for(int i = 0; i < str.Length / 2; i++)

        {

            str = Change(str, i);

        }

        return str;

    }

    public static string Change(string string_str, int i)

    {

        char temp;

        char[] str = string_str.ToArray();

        temp = str[str.Length - 1 - i];

        str[str.Length - 1 - i] = str[i];

        str[i] = temp;

        return new string(str);     

    }

}