ABC143B - TAKOYAKI FESTIVAL 2019

問題：B - TAKOYAKI FESTIVAL 2019

#include <iostream>
#include <vector>
using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)

int main(void) {
    int n, sum = 0;

    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> n;
    vector<int> d(n);
    REP(i, n) {
        cin >> d[i];
    }
    REP(i, n) {
        FOR(j, i + 1, n) {
            sum += d[i] * d[j];
        }
    }

    cout << sum << "\n";
    return 0;
}