SDNU 1272.SL的秘密

#include <cstdio>
#include <iostream>
#include <cmath>
#include <string>
#include <cstring>
#include <algorithm>
#include <queue>
#include <vector>
#include <map>
using namespace std;
#define ll long long
const int maxn = 5100;

ll n, sum;

ll qsm(ll a)
{
    ll ans = 1;
    for(int i = 1; i <= a; i++)
    {
        ans = ((ans%2009)*(i%2009))%2009;
    }
    return ans;
}

int main()
{
    while(~scanf("%lld", &n))
    {
        if(n<41)
        {
            sum = qsm(n);
            printf("%lld\n", sum);
        }
        else printf("0\n");
    }
    return 0;
}