#include <bits/stdc++.h>

using namespace std;

signed main() {

int n;
cin >> n;
int a[100001];
int sum=0;
for (int i = 1;i <= n;i++)
{
	cin >> a[i];
}
for (int i = 1;i <= n;i++)
{
	for (int j = i+1;j <= n;j++)
	{
		sum+=a[i]*a[j];
	}
}
cout << sum;
return 0;

}