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++ (GCC 9.2.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;
using ll=long long;
int main(void){
  int n;
  cin>>n;
  map<int,int> m;
  for(int i=0;i<n;i++){
    int a;
    cin>>a;
    m[a]++;
  }
  ll n1,n2;
  for(auto i:m){
    if(i.second>=4){
      n1=i.first,n2=i.first;
    }else if(i.second>=2){
      swap(n1,n2);
      n1=i.first;
    }
  }
  cout<<n1*n2<<endl;
  return 0;
}