Particle Accelerator II

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

int main() {
    int n;
    cin >> n;

    int aux;
    long long sum = 0;
    long long sumSq = 0;
    for (int i = 0; i < n; ++i) {
        cin >> aux;
        sum += aux;
        sumSq += aux * aux;
    }

    long long total = (sum * sum - sumSq) / 2;
    cout << total << endl;
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED