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: Wrong Answer)

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

int main() 
{
    int N;
    cin >> N;
    map<long long, int, greater<long long>> mp;
    for(int i = 0; i < N; i++){
        long long hoge = 0;
        cin >> hoge;
        mp[hoge]++;
    }
    
    long long ans = 0;
    long long ans1 = 0, ans2 = 0;
    int count = 0;
    bool flag = false;
    for (auto x : mp) {
        //cout << x.first << " : " << x.second << endl;
        if (x.second >= 4) {
            long long a = x.first*2;
            ans = max(ans, a);
            flag = true;
        }
        if (x.second >= 2 && count == 0) {
            ans1 = x.first;
            count++;
        }else if (count == 1){
            ans2 = x.first;
            count++;
            long long a = ans1 * ans2;
            ans = max(ans, a);
            break;
        }
    }
    
    if (count == 2 || flag) {
        cout << ans << endl;
    }else{
        cout << "0" << endl;
    }
}