Encrypt the Messages

// Encrypt 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. Your task is to write a program, which encrypts all messages in a //specific communication, prints them at the console as well as the total number of messages that have been sent.
//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 //encrypted messages’ content and total count. Any non-empty string between the “start” and “end” keywords is considered a message. //If no messages have been sent between the “start” and the “end” keywords, you should print on the console: “No messages sent.”
//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: ‘ ’ (space) is converted to a plus sign (‘ +’), ‘,’ is converted to ‘%’, ‘.’ is converted to ‘&’, //‘?’ is converted to ‘#’ and ‘!’ is converted to ‘$’. Digits (0-9) are not converted and stay the same.
//For example, you receive the following message –  “Secret message 1!” and you start encrypting it. Convert the 1st character ‘!’ //to ‘$’, then the 2nd character – ‘1’ stays the same, then covert the 3rd character – ‘ ’ to ‘+’, ‘e’  ’r’, ‘g’  ‘t’, ‘a’  ‘n’, //‘s’  ‘f’, ‘s’  ‘f’, ‘e’  ’r’ , ‘m’  ’z’, ‘ ’  ‘+’, ‘t’  ‘g’, ‘e’  ’r’ , ‘r’  ’e’ , ‘c’  ’p’ , ‘e’  ’r’ , ‘S’  ’F’. //After encrypting all letters, the message is: “Frperg+zrffntr+1$” and when you reverse it, you get the final encrypted message: //“$1+rtnffrz+greprF”

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

namespace ConsoleApplication5
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            string command = Console.ReadLine();
            StringBuilder digits = new StringBuilder();
            string result = null ;
            for (int i = 0; i < input.Length; i++)
            {
                int ascii = input[i];
                int lastDigit = ascii % 10;
                result = digits.Insert(i, lastDigit).ToString();
            }
            int number = 0;
            StringBuilder final = new StringBuilder ();
            string finalResult = null;
            int index = 0;
            for (int j = 0; j < result.Length;j++)
            {
                if ((result[j] - '0') % 2 == 0)
                {
                    number = (result[j] - '0') * (result[j] - '0');
                }
                else if ((result[j] - '0') % 2 != 0 && j != 0 && j != (result.Length - 1))
                {
                    number = (result[j] - '0') + (result[j - 1] - '0') + (result[j + 1] - '0');
                }
                else if ((result[j] - '0') % 2 != 0 && j == 0 && input.Length == 1)
                {
                    number = (result[j] - '0') + 0 + 0;
                }
                else if ((result[j] - '0') % 2 != 0 && j == 0)
                {
                    number = (result[j] - '0') + 0 + (result[j + 1]- '0');
                }
                else if ((result[j] - '0') % 2 != 0 && j == (result.Length - 1))
                {
                    number = (result[j] - '0') + 0 + (result[j - 1] - '0');
                }
                if (number > 9)
                {
                    finalResult = final.Insert(index, number).ToString();
                    index+=2;
                }
                else
                {
                    finalResult = final.Insert(index, number).ToString();
                    index++;
                }
            }
            if (command == "\\")
            {
                for (int f = 0; f < finalResult.Length; f++)
                {
                    for (int s = 0; s < finalResult.Length; s++)
                    {
                        if (f == s)
                        {
                            Console.Write(finalResult[f] + " ");
                        }
                        else
                        {
                            Console.Write("0" + " ");
                        }
                    }
                    Console.WriteLine();
                }
            }
            else if (command == "/")
            {
                int lett = finalResult.Length - 1;
                for (int f = 0; f < finalResult.Length; f++)
                {
                    for (int s = finalResult.Length - 1; s >= 0; s--)
                    {
                        if (f == s)
                        {
                            Console.Write(finalResult[lett] + " ");
                            lett--;
                        }
                        else
                        {
                            Console.Write("0" + " ");
                        }
                    }
                    Console.WriteLine();
                }
            }
        }
    }
}