#include <iostream>

using namespace std;
const int N = 2e5;
int a[N];
long long sum[N];

int main()
{
	int n; std :: cin >> n;
	for (int i = 1; i <= n; i ++ )
	{
		std :: cin >> a[i];
		sum[i] = sum[i - 1] + a[i];
	}
	long long s = 0;
	
	for (int i = 1; i <= n; i ++ )
	{
		s = s + a[i] * (sum[n] - sum[i]);
	}
	std :: cout << s;
	
	return 0;
}