decrypt message

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

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            string inputString;
            int counter = 0;
            int checker = 0;
            List<char> letters = new List<char>();
            List<string> output1 = new List<string>();
            do
            {
                inputString = Console.ReadLine();
            }
            while ((inputString.ToUpper() != "START"));
            inputString = Console.ReadLine();
            while ((inputString.ToUpper() != "END"))
    {


                char toBeConverted;
                for (int i = 0; i < inputString.Length; i++)
            {
             toBeConverted = inputString[i];
                    if ((toBeConverted >= 'A' && toBeConverted <= 'M') || (toBeConverted >= 'a' && toBeConverted <= 'm'))
    {
        int helpInt = Convert.ToInt32(toBeConverted);
        helpInt += 13;
                        toBeConverted = Convert.ToChar(helpInt);
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
    }
                    if ((toBeConverted >= 'N' && toBeConverted <= 'Z') || (toBeConverted >= 'n' && toBeConverted <= 'z'))
                    {
                        int helpInt = Convert.ToInt32(toBeConverted);
                        helpInt -= 13;
                        toBeConverted = Convert.ToChar(helpInt);
                        checker++;
                        letters.Add(toBeConverted);
                        continue;


                    }
                    if (toBeConverted == '+')
                    {
                        toBeConverted = ' ';
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted == '%')
                    {
                        toBeConverted = ',';
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted == '$')
                    {
                        toBeConverted = '!';
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted == '&')
                    {
                        toBeConverted = '.';
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted == '#')
                    {
                        toBeConverted = '?';
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted >= '0' && toBeConverted<= '9')
                    {
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted == ',')
                    {
                        toBeConverted = '%';
                       letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
                    if (toBeConverted == ' ')
                    {
                        toBeConverted = '+';
                        letters.Add(toBeConverted);
                        checker++;
                        continue;
                    }
            }
                letters.Reverse();
                if (checker > 0)
                {
                    counter++;
                    string s = string.Join("", letters);
                    letters = new List<char>();
                    output1.Add(s);
                    checker = 0;
                }
                inputString = Console.ReadLine();
    }
            if (counter > 0)
            {
              Console.WriteLine("Total number of messages: {0}", counter);
            foreach (var item in output1)
            {
                Console.WriteLine(item);
            }
            }
            else
            {
                Console.WriteLine("No message received.");
            }
        }
    }
}