HDU 2132 An easy problem

#include <bits/stdc++.h>
using namespace std;

const int maxn = 1e5 + 10;
long long sum[maxn];

int main() {
    sum[0] = 0;
    for(long long i = 1; i < maxn; i ++) {
        if(i % 3 == 0)
            sum[i] = sum[i - 1] + i * i * i;
        else
            sum[i] = sum[i - 1] + i;
    }
    int x;
    while(~scanf("%d", &x)) {
        if(x < 0)
            break;
        else
            printf("%lld\n", sum[x]);
    }
    return 0;
}