using System;

public class Program

{

    public static void Main()

    {

        start:

        Console.Clear();

        Console.WriteLine("Enter a positive whole number please");

        string input = Console.ReadLine();

        ulong n;

        if (ulong.TryParse(input , out n))

        { //opens tryparse 

        // if statment telling the user to enter a number smaller than whatever the max is 

        ulong fact = 1;

        for (ulong i = 1; i <= n; i++)

        {

            fact*= i;

        }

        Console.WriteLine("n! = " + fact);

        Console.WriteLine("Would you like to run the program again? y/n");

        string choice = Console.ReadLine();

        if (choice == "y" || choice == "Y")

            goto start;

        }// closes tryparse 

        else

        {Console.WriteLine("You must enter a positive whole number. Please try again.");

                        goto start; 

        }

    }

}

​