#include <stdio.h>

int FAC(int n)
{
	if( n < 0)
		return -1;
	if( n == 0)
		return n;
	return n * FAC(n-1);
}

int main(int argc, char** argv)
{
	int n;
	scanf("%d", &n);
	int ret;
	ret = FAC(n);
	if(ret == -1)
		printf("error\n");
	printf("%d\n", ret);
}