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

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

  vector<int> A(N, 0);
  for (int i; i < N; i++) {
    cin >> A[i];
  }

  sort(A.begin(), A.end());

  int x = 0, y = 0;
  int same = 0;

  for (int i = A.size() - 1; i > 0; i--) {
    if (A[i - 1] == A[i]) {
      if (x == 0) {
        x = A[i];
        same = 2;
      }
      else {
        if (same == 0) {
          y = A[i];
          break;
        }
        if (same == 2) {
          same = 3;
        }
        else if (same == 3) {
          y = A[i];
          break;
        }
      }
    }
    else {
      same = 0;
    }
  }

  cout << x * y << endl;
}