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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <map>
#include <vector>
using namespace std;
typedef long long int ll;
int main(int argc, char const *argv[]) {
  int N;
  cin >> N;
  vector<ll> A(N);
  for (int i = 0; i < A.size(); i++) {
    cin >> A[i];
  }
  sort(A.rbegin(), A.rend());
  int a = 0;
  int b = 0;
  for (int i = 0; i < A.size() - 1; i++) {
    if (A[i] == A[i + 1]) {
      if (a == 0) {
        a = A[i];
        i++;
        continue;
      } else if (b == 0) {
        b = A[i];
        break;
      }
    }
  }
  cout << a * b << endl;

  return 0;
}