import structure5.*;

class Factorial {
	public static int fact(int n) {
		Assert.pre(n >= 0, "n should be non-negative");
		if (n == 0) {
			return 1;
		}
		return n * fact(n - 1);
	}

	public static void main(String[] args) {
		int n = Integer.valueOf(args[0]);
		System.out.println(fact(n));
	}
}