/*
 * compute n!
 * *
 * Matt Bishop, ECS 36A, Fall 2019
 * version 1.1:	added error handling
 */
#include <stdio.h>

int nfact(int n)
{
	int x;
	
	if (n == 0)
		return(1);

	x = nfact(n-1);

	return(n * x);
}

int main(int argc, char *argv[])
{
	int n;

	if (argc != 2 || sscanf(argv[1], "%d", &n) != 1){
		fprintf(stderr, "Usage: %s [ number ]\n", argv[0]);
		return(1);
	}

	if (n >= 0)
		printf("%d! = %d\n", n, nfact(n));
	else{
		fprintf(stderr, "n! is only defined for non-negative integers\n");
		return(1);
	}
	

	return(0);
}