#include<iostream>
using namespace std;
int main()
{
    long long n,a[100008]={},s[100008]={},ans = 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++)
    {
        ans += a[i] * (s[n] - s[i]);
    }
    cout << ans ;
    
    return 0;
}