Как добавить факториал в нужное поле? - C#

static void Factorial_calculation()

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace HelloOperators_stud
{
    class Program
    {
        static void Main(string[] args)
        {
            long a;
            Console.WriteLine(@"Please,  type the number:
            1. Farmer, wolf, goat and cabbage puzzle
            2. Console calculator
            3. Factirial calculation
            ");
 
            a = long.Parse(Console.ReadLine());
            switch (a)
            {
                case 1:
                    Farmer_puzzle();
                    Console.WriteLine("");
                    break;

                case 2:
 
                    Calculator();
                    Console.WriteLine("");
                    break;
 
                case 3:
 
                    Factorial_calculation();
                    Console.WriteLine("");
                    break;
                default:
                    Console.WriteLine("Exit");
                    break;
            }
            Console.WriteLine("Press any key");
            Console.ReadLine();
        }
 
        private static void Factorial_calculation()
        {
            throw new NotImplementedException();
        }
        #region farmer
        static void Farmer_puzzle()
        {
 
        }
        #endregion
 
        #region calculator
        static void Calculator()
        {
            while (true)
            {
                {
                    double X1, X2;
                    string _operator;
                    Console.WriteLine("введите первое число: ");
                    X1 = Convert.ToDouble(Console.ReadLine());
                    Console.WriteLine("введите знак:+ или - или / или * или ^ ");
                    _operator = Console.ReadLine();
                    Console.WriteLine("Введите второе число: ");
                    X2 = Convert.ToDouble(Console.ReadLine());
                    switch (_operator)
                    {
                        case "+":
                            Console.WriteLine("otvet");
                            Console.WriteLine(X1 + X2);
                            break;
                        case "-":
                            Console.WriteLine("otvet");
                            Console.WriteLine(X1 - X2);
                            break;
                        case "*":
                            Console.WriteLine("otvet");
                            Console.WriteLine(X1 * X2);
                            break;
                        case "/":
                            Console.WriteLine("otvet");
                            Console.WriteLine(X1 / X2);
                            break;
                        case "^":
                            Console.WriteLine("otvet");
                            Console.WriteLine(Math.Pow(X1, X2));
                            break;
                    }

                }
                #endregion
            }
                #region Factorial
           
       {
                                             [COLOR="Red"]<=сюда впихнуть код[/COLOR]
                }
                #endregion
            }
        }
    }