ACM 11401 - Triangle Counting

#include <stdio.h>

unsigned long long int ans[1000001];

void create()
{
    int i;
    unsigned long long int s, b, now;
    ans[3] = 0;
    ans[4] = 1;
    for (i = 5; i < 1000001; i ++)
    {
        s = i;
        b = i / 2 + 1;
        now = s*(s-1)-(s+1)*(s-b)-b*(b-1);
        ans[i] = ans[i - 1] + now;
    }
}

int main()
{
    int n;
    create();
    while (scanf("%d", &n) == 1 && n >= 3)
        printf("%lld\n", ans[n]);
    return 0;
}

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