#include<iostream>
#include<algorithm>
using namespace std;
const int N=2e6+50;
typedef long long LL;
LL a[N];
int main(){
LL n,sum=0,res=0;
cin>>n;
for(int i=0;i<n;i++) cin>>a[i],sum+=a[i];
for(int i=0;i<n;i++) {
res+=a[i]*(sum-a[i]);
}
cout<<res/2<<endl;
return 0;
}