using System;

public class Program

{

    public static void Main()

    {

        start:

        Console.Clear();

        Console.WriteLine("Please choose an option from the menu:");

        Console.WriteLine("If you would like to calculate n!, please press 1.");

        Console.WriteLine("If you would like to calculate nPr, please press 2.");

        Console.WriteLine("If you would like to calculate nCr, please press 3.");

​

        int choice;

        if (int.TryParse(Console.ReadLine(),out choice))

        {

            Console.WriteLine("");

            goto start;

        }

​

        switch (choice)

        {

            case 1:

                Console.Clear();

                Console.WriteLine("Please enter your value:");

                long num = long.Parse(Console.ReadLine());

                Console.WriteLine("The factorial of " + num + " = " + Fact(num));

                break;

​

            case 2:

                Console.Clear();

                Console.WriteLine("Please enter your first value:");

                long n = long.Parse(Console.ReadLine());

                Console.WriteLine("Please enter your second value:");

                long r = long.Parse(Console.ReadLine());

                Console.WriteLine(n + "P" + r + " = " + nPr(n, r));

                break;

​

            case 3:

                Console.Clear();

                Console.WriteLine("Please enter your first value:");

                long nC = long.Parse(Console.ReadLine());

                Console.WriteLine("Please enter your second value:");

                long rC = long.Parse(Console.ReadLine());

                Console.WriteLine(nC + "C" + rC + " = " + nCr(nC, rC));

                break;

​

            default:

                Console.WriteLine("");

                goto start;

        }

​

        Console.ReadLine();

    }

​

    static public long Fact(long a)

    {

        long fact = 1;

        for (int i = 1; i <= a; i++)

        {

            fact *= i;

        }

        return fact;

    }

​

    static public long nPr(long n, long r)

    {

        return Fact(n) / Fact(n - r);

    }

​

    static public long nCr(long n, long r)

    {

        return Fact(n) / (Fact(r) * Fact(n - r));

    }

}