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: Wrong Answer)

#include <iostream>
#include <queue>
#include <map>
using namespace std;
int main(void) {
  long long n, a;
  map<long long, bool> used;
  priority_queue<long long> que;

  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a;
    if (used[a]) {
      used[a] = false;
      que.push(a);
    } else {
      used[a] = true;
    }
  }

  if (que.size() < 2) {
    cout << 0 << endl;
    return 0;
  }

  int x = que.top(); que.pop();
  int y = que.top();
  cout << x * y << endl;

  return 0;
}