Encrypt the Messages

using System;


namespace EncryptTheMessages
{
    class EncryptTheMessages
    {
        static void Main()
        {
            int numberOfMsg = 0;
            string result = "";

            string start = Console.ReadLine();
            while (start != "START" && start != "start")
            {
                start = Console.ReadLine();
            }
            string input = Console.ReadLine();

            while (input != "END" && input != "end")
            {
                if (string.IsNullOrWhiteSpace(input))
                {
                    input = Console.ReadLine();
                    continue;
                }
                result += EncryptedWord(input);
                result += Environment.NewLine;
                input = Console.ReadLine();
                if (input == "End" && input == "end")
                {
                    break;
                }
                numberOfMsg += Message(0);
            }
            if (numberOfMsg != 0)
            {
                Console.WriteLine("Total number of messages: {0}",numberOfMsg);
                Console.WriteLine(result);
            }
            else
            {
                Console.WriteLine("No messages sent.");
            }

        }

        static private string EncryptedWord(string word)
        {
            string encryptedMsg = "";
            int charDifference = 13;
            for (int i = word.Length - 1; i >= 0; i--)
            {
                if (char.IsLetter(word[i]))
                {
                    if ((word[i] >= 'A' && word[i] <= 'M') || (word[i] >= 'a' && word[i] <= 'm'))
                    {
                        int swap = Convert.ToInt32(word[i]) + charDifference;
                        encryptedMsg += Convert.ToChar(swap);
                    }
                    else if ((word[i] >= 'N' && word[i] <= 'Z') || (word[i] >= 'n' && word[i] <= 'z'))
                    {
                        int swap = Convert.ToInt32(word[i]) - charDifference;
                        encryptedMsg += Convert.ToChar(swap);
                    }
                }
                else
                {
                    switch (word[i])
                    {
                        case ' ': encryptedMsg += '+'; break;
                        case ',': encryptedMsg += '%'; break;
                        case '.': encryptedMsg += '&'; break;
                        case '?': encryptedMsg += '#'; break;
                        case '!': encryptedMsg += '$'; break;
                        default: encryptedMsg += word[i]; break;
                    }

                }
            }
            if (!(string.IsNullOrEmpty(encryptedMsg)))
            {
                 Message(0);
            }
            return encryptedMsg;
        }

        static private int Message(int msg)
        {
            msg++;
            return msg;
        }
    }
}