#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n; cin >> n;
    map<int, int> m;
    vector<int> v(0);
    for (int i = 0; i < n; i++)
    {
        int be; cin >> be;
        m[be]++;
    }
    for (auto it = m.begin(); it != m.end(); ) {
        if (it->second < 2) {
            it = m.erase(it);
        } else {
            v.push_back(it->first);
            ++it;
        }
    }
    sort(v.begin(), v.end());
    if (v.size() > 0 && m[v.size()-1] > 3)
    {
        cout << v[v.size()-1] * v[v.size()-1] << endl;
    }
    else
    {
        if (v.size() > 1)
        {
            cout << v[v.size()-1] * v[v.size()-2] << endl;
        }
        else
        {
            cout << "0" << endl;
        }
    } 
    return 0;
}