$$ 0*a_1 + a_1 * a_2 + ( a_1 + a_2 ) * a_3 + ( a_1 + a_2 + a_3 ) * a_4 $$

#include<iostream>
using namespace std;
 
int main()
{
	long long int n, temp, sum=0, ans=0;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		cin >> temp;
		ans += sum*temp;
		sum += temp;
	}
	cout << ans;
	return 0;
}