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: Time Limit Exceeded)

#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <algorithm>
#include <utility>

using namespace std;

bool binarySearch(vector<int> A, int s, int e, int n){
  int half = (s + e) / 2;
  if(s == half){
    if(A[s] == n || A[e] == n) return true;
    else return false;
  }
  
  if(A[half] == n) return true;
  if(A[half] < n) return binarySearch(A, half, e, n);
  if(A[half] > n) return binarySearch(A, s, half, n);

  return false;
}

void show(vector<int> A){
  for(int i = 0; i < A.size(); i++){
    cout << A[i] << " ";
  }
  cout << endl;
}

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

  sort(A.begin(), A.end());
  vector<int> ans;
  
  for(int i = A.size() - 1; i > 0; i--){
    bool flag = binarySearch(A, 0, i - 1, A[i]);
    if(flag){
      if(ans.size() != 0){
        bool searched = false;
        for(int j = 0; j < ans.size(); j++){
          if(ans[j] == A[i]) searched = true;
        }
        if(!searched) ans.push_back(A[i]);
      }
      else ans.push_back(A[i]);
    }
  }

  if(ans.size() < 2) cout << "0" << endl;
  else cout << ans[0] * ans[1] << endl;
  
  return 0;
}