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;
using ll=long long;

int main(){
  ll N; cin>>N;
  map<ll, ll> R;
  for(ll i=0; i<N; i++){
    ll A; cin>>A;
    if(R.count(A)) R[A]++;
    else R[A]=1;
  }
  priority_queue<int> pq;
  ll count=0;
  for(auto p:R){
    auto k=p.first;
    auto v=p.second;
    if(v>=2){
      while(v>=2){
        pq.push(k);
        v=v/2;
      }
      count++;
    }
  }
  if(count<2) cout<<0<<endl;
  else{
    ll ans=pq.top();
    pq.pop();
    cout<<ans*pq.top()<<endl;
  }
}