简单站外题求助

#include <iostream>
using namespace std;

const int N = 1000001;

int main() {
    int n;
    cin >> n;
    
    if (n < 3) {
        cout << 0;
        return 0;
    }
    
    long long a[N] = {};
    a[0] = 1;
    for (int i = 1; i <= n-2; i++) {
        a[i] = i * (i + 1) / 2;
    }
    
    cout << a[n-2];
    
    return 0;
}