/*
    Factorial.java
    CIS 160
    David Klick
    2007-10-30

    Calculates the factorial of integers from 0 through 20.
*/

import java.util.Scanner;

public class Factorial {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = 0;
        do {
            System.out.print("Enter a number (0-20) you want the factorial of (-1 to quit): ");
            n = in.nextInt();
            if (n >= 0 && n <= 20) {
                System.out.println(n + "! = " + fact(n));
            } else if (n != -1) {
                System.out.println("Error: Invalid input");
            }
        } while (n != -1);
    }
   
    private static long fact(long n) {
        if (n < 2) {
            return 1;
        } else {
            return n * fact(n-1);
        }
    }
}