#include <bits/stdc++.h>

using namespace std;

using iter = vector<int>::const_iterator;

long value(iter start, iter end)
{
    long res = 0;
    while(start < end - 1)
    {
        iter next = start + 1;
        while(next < end)
        {
            res += *start * *next;
            ++next;
        }
        ++start;
    }
    return res;
}

long largestValue(const vector<int>& a)
{
    long max = LONG_MIN;
    for (iter i = a.begin(); i < a.end() - 1; ++ i)
    {
        for (iter j = i + 1; j < a.end(); ++j)
        {
            long cur = value(i, j);
            if(cur > max)
                max = cur;
        }
    }
    return max;
}

int main() {
    int n;
    cin >> n;
    vector<int> A(n);
    for(int A_i = 0; A_i < n; A_i++){
       cin >> A[A_i];
    }
    long result = largestValue(A);
    cout << result << endl;
    return 0;
}