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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <map>
#include <vector>
using namespace std;

int main() {
  int n;
  cin >> n;


  map <int, int, greater<int>> seen;

  for (int i = 0; i < n; ++i) {
    int x;
    cin >> x;

    ++seen[x];
  }

  vector <long long> ans;
  for (auto it = seen.begin(); it != seen.end(); ++it) {
    if (ans.size() >= 2)
      break;
    if (it->second >= 4) {
      ans.push_back(it->first);
      ans.push_back(it->first);
    } else if (it->second >= 2) {
      ans.push_back(it->first);
    }
  }

  if (ans.size() < 2) {
    cout << 0 << endl;
  } else {
    long long answer = ans[0] * ans[1];
    cout << answer << endl;
  }
}