Decrypt the Messages

// Decrypt the Messages
//You are working for a company which is very concerned about its information and communication. For this reason, they have //invented an internal approach to communication between different departments – they are communicating to each other via messages, //which are reversed (written backwards) and then encrypted. In order to be read and understood, each message has to be decrypted. //Your task is to write a program, which receives all encrypted messages in a specific communication, decrypts them and prints all //decrypted messages at the console as well as the total number of messages that have been received.
//At the beginning of a communication, you will receive either the keyword “START” (upper case) or “start” (lower case), which //indicates that you will start receiving reversed and encrypted messages. At the end of the communication, you will receive either //the keyword “END” (upper case) or “end” (lower case), which indicates that the communication is over and you need to show the //decrypted messages’ content and total count. Any non-empty string between the “start” and “end” keywords is considered a message. //If no messages have been received between the “start” and the “end” keywords, you should print on the console: “No message //received.”
//All messages are case-sensitive and consist of letters, digits, as well as some special characters – ‘+’, ‘%’, ‘&’, ‘#’ and ‘$’. //Letters from A to M are converted to letters from N to Z (A  N; B  O; … M  Z) and letters from N to Z are converted to letters //from A to M (N  A; O  B; … Z  M). The converted letter should keep the case of the original letter. The special characters are //converted in the following way: ‘+’ is converted to a single space (‘  ’), ‘%’ is converted to a comma (‘,’), ‘&’ is converted to //a dot (‘.’), ‘#’ is converted to a question mark (‘?’) and ‘$’ is converted to an exclamation mark (‘!’). The digits (0-9) are //not converted and stay the same.
//For example, you receive the following message – “$1+rtnffrz+greprF” and you start decrypting it. Convert the 1st character ‘$’ //to ‘!’, then the 2nd character – ‘1’ stays the same, then covert the 3rd character – ‘+’ to single space ‘ ’, ‘r’  ’e’, ‘t’  //‘g’, ‘n’  ‘a’, ‘f’  ‘s’, ‘f’  ‘s’, ‘r’  ’e’ , ‘z’  ’m’, ‘+’  ‘ ’, ‘g’  ‘t’, ‘r’  ’e’ , ‘e’  ’r’ , ‘p’  ’c’ , ‘r’  ’e’ //, ‘F’  ’S’. After decrypting all letters, the message is: “!1 egassem terceS” and when you reverse it, you get the original //message: “Secret message 1!”


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

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {

            string input;
            StringBuilder newInput = new StringBuilder();
            int p = 0;

            List<string> arr = new List<string>();

            int messages = 0;
            do
            {
                input = Console.ReadLine();
                if (input == "START" || input == "start")
                {
                    int lenght = input.Length;
                    p = 0;
                    newInput = new StringBuilder();

                    if (input != "end" && input != "END" && input != "start" && input != "START")
                    {
                        for (int i = 0; i < lenght; i++)
                        {
                            if (input[i] >= 'a' && input[i] <= 'm')
                            {
                                newInput.Insert(p, (char)(input[i] + 13));
                                p++;
                            }
                            else if (input[i] >= 'n' && input[i] <= 'z')
                            {
                                newInput.Insert(p, (char)(input[i] - 13));
                                p++;
                            }
                            else if (input[i] >= 'A' && input[i] <= 'M')
                            {
                                newInput.Insert(p, (char)(input[i] + 13));
                                p++;
                            }
                            else if (input[i] >= 'N' && input[i] <= 'Z')
                            {
                                newInput.Insert(p, (char)(input[i] - 13));
                                p++;
                            }
                            else if (input[i] == '+')
                            {
                                newInput.Insert(p, (char)(input[i] - 11));
                                p++;
                            }
                            else if (input[i] == '%')
                            {
                                newInput.Insert(p, (char)(input[i] + 7));
                                p++;
                            }
                            else if (input[i] == '#')
                            {
                                newInput.Insert(p, (char)(input[i] + 28));
                                p++;
                            }
                            else if (input[i] == '$')
                            {
                                newInput.Insert(p, (char)(input[i] - 3));
                                p++;
                            }
                            else if (input[i] == '&')
                            {
                                newInput.Insert(p, (char)(input[i] + 8));
                                p++;
                            }
                            else if (input[i] >= '0' && input[i] <= '9')
                            {
                                newInput.Insert(p, (char)(input[i]));
                                p++;
                            }

                        }
                        string text = newInput.ToString();
                        while ((input != "end" && input != "END") && lenght != 0)
                        {
                            arr.Add(text);
                            messages++;
                            break;
                        }
                    }
                    else
                    {
                        break;
                    }
                }
            } while (input != "end" || input != "END");
            if (messages == 0)
            {
                Console.WriteLine("No message received.");
            }
            else
            {
                Console.WriteLine("Total number of messages: {0}", messages);
            }

            for (int k = 0; k < arr.Count; k++)
            {
                string newInputt = arr[k].ToString();
                newInputt = Reverse(newInputt);
                Console.WriteLine(newInputt);
            }
        }
        public static string Reverse(string str)
        {
            char[] chars = str.ToCharArray();
            for (int i = 0, j = str.Length - 1; i < j; i++, j--)
            {
                char c = chars[i];
                chars[i] = chars[j];
                chars[j] = c;
            }
            return new string(chars);
        }
    }
}