Strings 1

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

namespace ConsoleApplication21
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Would you like to encrypt or decrypt a string?");
            string Choice = Console.ReadLine().ToLower();
            if (Choice[0] == 'e')
            {
                string Stnew = "";
                Console.WriteLine("Input a string");
                string St = Console.ReadLine().ToLower();

                for (int i = 0; i < St.Length; ++i)
                {

                    if (St[i] != 'z')
                    {
                        Stnew += (char)(St[i] + 1);
                    }
                    else
                    {
                        Stnew += 'a';
                    }
                }
                Console.WriteLine(Stnew);
            }
            else
            {

                string Stnew = "";
                Console.WriteLine("Input a string");
                string St = Console.ReadLine().ToLower();

                for (int i = 0; i < St.Length; ++i)
                {

                    if (St[i] != 'a')
                    {
                        Stnew += (char)(St[i] - 1);
                    }
                    else
                    {
                        Stnew += 'z';
                    }
                }
                Console.WriteLine(Stnew);
            }
        }
    }
}