#include <cassert>
#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for(int i = 0; i < (int)(n); i++)
constexpr int inf = 2e9;
/*
#include <atcoder/all>
using namespace atcoder;

#include <boost/multiprecision/cpp_int.hpp>
#include <boost/math/common_factor_rt.hpp>
namespace mp = boost::multiprecision;
*/
using ll = long long;
int main()
{
    int N;
    cin >> N;
    vector A(N,0);
    rep(i,N) cin >> A[i];
    ll ans = 0;
    ll now_size = A[0];
    for (int i = 1; i < N; i++)
    {
        ans += now_size * A[i];
        now_size += A[i];
    }
    cout << ans << endl;
}