using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(DecodingPswd(""));

    }

    public static string DecodingPswd(string OldPswd)

    {

        if (OldPswd == null)

        {

            return null;

        }

        if (OldPswd.StartsWith("B64-"))

        {

            OldPswd = new UTF8Encoding().GetString(Convert.FromBase64String(OldPswd.Substring(4)));

        }

        int length = OldPswd.Length;

        char[] chArray = new char[length];

        for (int i = 0; i < length; i++)

        {

            chArray[i] = OldPswd[i];

            int num3 = chArray[i];

            num3 -= (30 + i) + 1;

            chArray[i] = (char) num3;

        }

        int num4 = length % 2;

        if (num4 == 1)

        {

            int num5 = chArray[length - 1];

            num5 += 30 + length;

            chArray[length - 1] = (char) num5;

        }

        StringBuilder builder = new StringBuilder();

        for (int j = 0; j < length; j++)

        {

            builder.Append(chArray[j].ToString());

        }

        return builder.ToString();

    }

​

​

​

}