#include <stdio.h>                     
int factorial(int n);                   
int main(int argc, char const *argv[]) 
{
    int n; 
    scanf("%d", &n);
    if (n>0)
    {
    printf("%d\n", factorial(n));	
	}
	else
	{
	printf("请重新输入一个非负整数:");
    	scanf("%d", &n);
    printf("%d\n", factorial(n));
}
    return 0;
}
int factorial(int n)
{
    int fact = 1;

    for (int i = 1; i <= n; ++i)
    {
        fact = fact*i;
    }

    return fact;
}