Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

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

void solve() {
    int n;
    cin >> n;
    map<long long, long long> mp;
    mp[0] = 0;
    for (int i = 0; i < n; i++) {
        long long x;
        cin >> x;
        mp[x]++;
    }
    auto i = mp.end();
    vector<long long> ans(3);
    int cnt = 0;
    while(i != mp.begin() && cnt < 2) {
        while(mp[(*i).first] >= 2 && cnt < 2) {
            ans[cnt] = (*i).first;
            cnt++;
            mp[(*i).first] -= 2;
        }
        i--;
    }
    cout << ans[0] * ans[1];
}


int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    solve();
    return 0;
}