ckckck

using System;
using System.Text;
using System.Text.RegularExpressions;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var artistRegex = new Regex("^[A-Z][a-z'\\s]+$");
            var songsRegex = new Regex("^[A-Z\\s]+$");
            var input = Console.ReadLine();
            while (input != "end")
            {
                var inputAsArray = input.Split(':');
                var artist = inputAsArray[0];
                var song = inputAsArray[1];

                if (!artistRegex.IsMatch(artist) || !songsRegex.IsMatch(song))
                {
                    Console.WriteLine("Invalid input!");
                }
                else
                {
                    var key = artist.Length;
                    var output = new StringBuilder();

                    foreach (var artistChar in artist)
                    {
                        if (artistChar == ' ' || artistChar == '\'')
                            output.Append(artistChar);
                        else
                        {
                            output.Append(GetChar(artistChar, key));
                        }
                    }

                    output.Append('@');

                    foreach (var songChar in song)
                    {
                        if (songChar == ' ' || songChar == '\'')
                            output.Append(songChar);
                        else
                        {
                            output.Append(GetChar(songChar, key));
                        }
                    }

                    Console.WriteLine($"Successful encryption: {output.ToString()}");
                }
                input = Console.ReadLine();
            }


        }

        private static char GetChar(char ch, int key)
        {
            for (int i = 0; i < key; i++)
            {
                if (ch == 90)
                {
                    ch = (char) 65;
                }
                else if (ch == 122)
                {
                    ch = (char) 97;
                }
                else
                {
                    var currentAsciiCode = (int) ch;
                    currentAsciiCode++;

                    ch = (char) currentAsciiCode;
                }
            }

            return ch;
        }

    }
}