class factorial {
	/** @return n! */
	static long fact(int n) {
		// To fit n! in a long variable, require n < 21
		assert (n >= 0) && (n <= 20) : "n out of range";
		// Make a stack just big enough
		Stack<Integer> S = new AStack<Integer>(n);
		while (n > 1) S.push(n--);
		long result = 1;
		while (S.length() > 0)
			result = result * S.pop();

		return result;
	}

	public static void main(String[] args) {
		System.out.println(fact(5));
	}
}