#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    int n;
    cin >> n;
    ll s = 0;
    vector<ll> a(n);
    rep(i, n) {
        cin >> a[i];
        s += a[i];
    }
    ll ans = 0;
    rep(i, n) {
        ans += a[i] * (s - a[i]);
    }
    cout << ans / 2 << endl;
    return 0;
}