#include<bits/stdc++.h>
#define int long long
using namespace std;
signed main()
{
	int n , a[10000];
	int ans1 = 0 , ans2 = 0;
	cin >> n;
	for(int i = 1;i <= n;i++)
	{
		cin >> a[i];
		ans1 += a[i] * ans2;
		ans2 += a[i];
	}
	cout << ans1 << endl;
	return 0;
}