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

using ll = long long;

int main() {
	int N;
	cin >> N;
	vector<int> A(N);
	for(int i = 0; i < N; ++i) cin >> A[i];
	
	ll Asum = A[0], ans = 0;
	for(int i = 1; i < N; ++i) {
		ans += Asum * A[i];
		Asum += A[i];
	}
	
	cout << ans << endl;
}