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

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N; cin >> N;
    long long sum = 0,answer = 0;
    while(N--){
        int a; cin >> a;
        answer += sum*a,sum += a;
    }    
    cout << answer << endl;
}