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(){
  int i,j,cnt=0;
  int n;
  cin>>n;
  int a[n];
  for(i=0;i<n;i++)cin>>a[i];
  sort(a,a+n);
  ll prod=1;
  for(i=n-1;i>=1;i--){
    if(a[i]==a[i-1]){
      prod*=a[i];
      i--;
      cnt++;
    }
    if(cnt==2)break;
  }
  if(cnt==2)cout<<prod<<endl;
  else cout<<0<<endl;
}