#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#define ll long long 
using namespace std;


int main() {
    ll n ;
    cin>>n;
    vector<int>v(n,0);
    for(ll i=0;i<n;i++){
        cin>>v[i];
    }
    ll pro = 0;
    for(ll i=0;i<n-1;i++){
        for(ll j=i+1;j<n;j++){
            pro += v[i]*v[j];
        }
    }
    printf("%lld",pro);
    return 0;
}