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;
#define ALL(v) (v).begin(), (v).end()

int main()
{
  int N; cin >> N;
  map<int, int> mp;
  for (int i = 0; i < N; ++i) {
    int a; cin >> a;
    mp[a]++;
  }

  vector<int> edge;
  for (auto v : mp) {
    while (v.second >= 2) {
      v.second -= 2;
      edge.push_back(v.first);
    }
  }

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

  sort(ALL(edge), greater<int>());
  cout << edge[0] * edge[1] << endl;
}