using System;

using System.Collections.Generic;

namespace ConsoleApp2

{

    class Program

    {

        private static string Encrypt(string a)

        {

            List<char> list = new List<char>(a);

            for (int i = 0; i < a.Length; i++)

            {

                if (i % 2 == 0)

                {

                    if (Char.IsUpper(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 65 - 1);

                        if (ascii < 0)

                            ascii += 26;

                        list[i] = Convert.ToChar(ascii + 65);

                    }

                    else if (Char.IsLower(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 97 - 1) ;

                        if (ascii < 0)

                            ascii += 26;

                        list[i] = Convert.ToChar(ascii + 97);

                    }

                }

                else

                {

                    if(Char.IsUpper(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 65 + 3) % 26;

                        list[i] = Convert.ToChar(ascii + 65);

                    }

                    else if(Char.IsLower(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 97 + 3) % 26;

                        list[i] = Convert.ToChar(ascii + 97);

                    }

                }

            }

            string b = new string(list.ToArray());

            return b;

        }

        private static string Decrypt(string a)

        {

            List<char> list = new List<char>(a);

            for (int i = 0; i < a.Length; i++)

            {

                if (i % 2 == 0)

                {

                    if (Char.IsUpper(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 65 + 1) % 26;

                        list[i] = Convert.ToChar(ascii + 65);

                    }

                    else if (Char.IsLower(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 97 + 1) % 26;

                        list[i] = Convert.ToChar(ascii + 97);

                    }

                }

                else

                {

                    if (Char.IsUpper(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 65 - 3);

                        if (ascii < 0)

                            ascii += 26;

                        list[i] = Convert.ToChar(ascii + 65);

                    }

                    else if (Char.IsLower(list[i]))

                    {

                        int ascii = (Convert.ToInt32(list[i]) - 97 - 3);

                        if (ascii < 0)

                            ascii += 26;

                        list[i] = Convert.ToChar(ascii + 97);

                    }

                }

            }

            string b = new string(list.ToArray());

            return b;

        }

        static void Main(string[] args)

        {

            string x = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

            string y = Encrypt(x);

            string z = Decrypt(y);

            Console.WriteLine(x);

            Console.WriteLine(y);

            Console.WriteLine(z);

            

            Console.Write("Enter a TEXT to encrypt : ");

            string txt = Console.ReadLine();

            y = Encrypt(txt);

            z = Decrypt(y);

            Console.WriteLine("Encryption TEXT : " + y);

            Console.WriteLine("Decryption The Encrypted TEXT : " + z);

        }

    }

}