#include <stdio.h>

int main(){
	int n, i, f = 1;

	scanf("%d", &n);
	if(n<0){
		printf("-1\n");
		return 1;
	}

	i=1; 
	while(i<=n){
		f = f*i;
		i++;
	}

	printf("%d\n", f);
	
	return 0;
}