#include <iostream>
using namespace std;

long long a[200010];
long long s[200010];

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