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(){
  long long n,flag1=0,flag2=0,ta=0,yo=0;
  vector<long long> a;
  cin>>n;
  for(int i=0;i<n;i++){
    cin>>ta;
    a.push_back(ta);
  }
  ta=0;
  sort(a.begin(),a.end(),greater<long long>());
  for(int i=0;i<n;i++){
    if(flag1==0 && a[i]==a[i+1]){
      ta=a[i];
      i++;  
      flag1=1;
    }
    else if(a[i]==a[i+1]){
      yo=a[i];
      break;
    }
  }
  if(yo==0 || ta==0)
    cout<<0<<'\n';
  else
    cout<<ta*yo<<'\n';
  return(0);
}