D. Minimum Triangulation

Link

Solution

$\sum_{i=2}^{n-1}i*(i+1)$

Code

#include <iostream>
typedef long long LL;
using namespace std;

int main() {
    int n;
    LL ans = 0;
    cin >> n;
    for (int i = 2; i < n; i++)ans += i * (i + 1);
    cout << ans << endl;
    return 0;
}