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: Accepted)

#include <bits/stdc++.h>

using namespace std;

int main(){
  int N;
  cin >> N;
  map<int,int,greater<int>> M;
  for(int i=1; i<=N; i++){
    int A;
    cin >> A;
    M[A]++;    
  }
  vector<long long> ans;
  auto itr=M.begin();
  while(ans.size()<2){
    if(itr->second>=2) ans.emplace_back(itr->first);
    if(itr->second>=4) ans.emplace_back(itr->first);
    if(itr==M.end()) break;
    else itr++;
  }
  if(ans.size()<2) cout << 0 << endl;
  else cout << ans[0]*ans[1] << endl;
  
  return 0;
}