using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Input positive integer: ");

        string num = Console.ReadLine();

        int s;

        bool check = int.TryParse(num, out s);

        if(check){

            int x = 1;

            int sum = s;

            while(x < s){

                sum = sum * x;

                x ++;

            }

            Console.WriteLine($"Factorial of {s} is {sum}.");

        }else{

            Console.WriteLine("Invalid input.");

        }

    }

}