#include <iostream>
using namespace std;
const int N = 2e6 + 10;
int a[N];
int n;
long long s[N];
int main()
{
cin >> n;
long long isum = 0;
for(int i = 1;i <= n;i ++)
{
cin >> a[i];
s[i] = s[i - 1] + a[i];
}
for(int i = 1;i < n;i ++)
{
isum += a[i] * (s[n] - s[i]);
}
cout << isum << endl;
return 0;
}