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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            string output = decrept("D");
            Console.WriteLine(output);
            Console.Read();
        }

     
        public static string decrept(string cybertext)
        {
            string plaintecxt = "";
            string arr="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            int position;
            int keypotion;
            char ch;
            int inverse = 21;

            for(int i=0;i<cybertext.Length;i++)
            {
                position = arr.IndexOf(cybertext[i]);
                keypotion = inverse*(position - 3) % 26;
                ch = arr[keypotion];
                plaintecxt =plaintecxt+ ch;

            }

            return plaintecxt;
        }
    }
}