#include <bits/stdc++.h>
using namespace std;
int main()
{
	long sum = 0;
	size_t Q, i, j;
	cin >> Q;
	long * arr = new long[Q];
	
    for( i = 0; i < Q; ++i)
	{
		cin >> arr[i];
	}
	
	for( i = 0; i < Q; ++i)
	{
		for( j = i+1; j < Q; ++j)
		{
			sum += arr[i] * arr[j];
		}
	}
	cout<<sum;
	
}