using System;
using System.Linq;

public class Kata
{
  public static string Rot13(string message)
  {
    return new string(message.Select(Replacer).ToArray());
        }

        static char Replacer(char a)
        {
            var alphabet = new Alphabet();

            if (char.IsLetter(a))
            {
                int index = alphabet.IndexOfAlphabet(a);
                var newChar = alphabet[index + 13];
                return char.IsUpper(a) ? char.ToUpper(newChar) : newChar;
            }

            return a;
        }
    }

    public class Alphabet
    {
        private static string _alphabet = "abcdefghijklmnopqrstuvwxyz";

        public char this[int i] => i >= _alphabet.Length ? _alphabet[i - _alphabet.Length] : _alphabet[i];

        public int IndexOfAlphabet(char a) => _alphabet.IndexOf(char.ToLower(a));
    }