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>
#define all(x) (x).begin(), (x).end()
using namespace std;
typedef long long ll;
const int MOD = 1e9 + 7;

int main() {
  int n;
  cin >> n;
  map<int, int> a;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    a[x]++;
  }
  multiset<int> list;
  for (auto i = a.begin(); i != a.end(); i++) {
    int p = (*i).second;
    if (p >= 2) list.insert((*i).first);
    if (p >= 4) list.insert((*i).first);
  }
  cout << (list.size() >= 2 ? (*list.rbegin()) * (*list.rbegin() - 1) : 0) << endl;
  return 0;
}