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

void solve(){
  using ll=long long;
  int n;
  cin>>n;
  vector<ll> a(n);
  ll ans=0,sum=0;
  for (int i=0;i<n;i++) cin>>a[i],sum+=a[i],ans-=a[i]*a[i];
  cout<<(ans+=sum*sum)/2<<endl;
}

int main(){
  int t=1;
  //cin>>t;
  while (t--) solve();
}