/*
    author  : real_rpg  
    country : India
*/
 
#include "bits/stdc++.h" 
#define ll long long
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define fl(i,a,b,c) for(long long i=a; i<b; i+=c)
#define rfl(i,a,b,c) for(long long i=a; i>=b; i-=c) 
#define rep(i,n) fl(i,0,n,1)
#define rrep(i,n) rfl(i,n-1,0,1)
#define sas int tt; cin >> tt; rep(tc,tt) 
#define pb  push_back
#define F first
#define S second
using namespace std; 
const int md = 1e9 + 7;
const int MAX = 1e5 + 5;
const int MAXN = 1e6 + 5;

void solve() {
	ll n; cin>>n;
	ll maxi=-1*INT_MAX;
	ll a[n]; rep(i,n) {
		cin>>a[i];
	}
	ll ans=0;
	fl(i,0,n-1,1) {
		fl(j,i+1,n,1) {
			ans += a[i]*a[j];
		}
	} cout<<ans;

} 
 
int main() { 
        ios_base::sync_with_stdio(false); 
        cin.tie(NULL); cout.tie(NULL); 
        solve(); 
        return 0; 
}