String encryption

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

namespace encription
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string cypher = string.Empty;
            for (int i = 0; i < n; i++)
            {
                char letter = char.Parse(Console.ReadLine());
                cypher+=Encrypt(letter);
            }
            Console.WriteLine(cypher);
        }
        static string Encrypt(char letter)
        {
            int asciiCode = letter;
            int appendFirstLast = asciiCode;
            string append = string.Empty;
            string midlle = string.Empty;
            if (asciiCode >= 100)
            {
                while (appendFirstLast > 0)
                {
                    append = (appendFirstLast % 10).ToString();
                    midlle = midlle.Insert(0, append);
                    appendFirstLast /= 100;
                }
            }
            else if (asciiCode < 100)
            {
                while (appendFirstLast > 0)
                {
                    append = (appendFirstLast % 10).ToString();
                    midlle = midlle.Insert(0, append);
                    appendFirstLast /= 10;
                }
            }
            int intMidlle = int.Parse(midlle);
            char start = (char)(asciiCode + (intMidlle % 10));
            char end = (char)(asciiCode - ((intMidlle / 10) % 10));
            string cypher = start + midlle + end;
            return cypher;
        }
    }
}