using System;

public class Program

{

     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 = int.Parse(Console.ReadLine());

            if (choice == 1)

                 {  Console.Clear();

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

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

                    Console.WriteLine("The Factorial of " + ans + "= " + Fact(ans));

                 }

            else if  (choice == 2)

                {   Console.Clear();

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

                    long num1 = long.Parse(Console.ReadLine());

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

                    long num2 = long.Parse(Console.ReadLine());

                    Console.WriteLine(num1+ "P"  + num2 + "= " + nPr(num1, num2));

                }

            else

            {Console.WriteLine("Please Enter a Valid Choice");

            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)

            {   long perm = Fact(n) / Fact(n - r);

                return perm;

            }

}

​