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;
#define long long long

int main(){
  int N;
  vector<long> A;
  cin >> N;
  for(int i=0; i<N; i++){
    long a; cin >> a;
    A.push_back(a);
  }
  sort(A.begin(), A.end(), greater<int>());
  int n=4;
  long tmp=A[0];
  long num=1;
  long ans=1;
  for(int i=1; i<N; i++){
    if(tmp==A[i]){
      num++; continue;
    }
    if(num>=2){
      if(n==4 && num>=4){
        cout << tmp*tmp << endl;
        return 0;
      }else if(n==2 && num>=2){
        ans*= tmp;
        cout << ans << endl;
        return 0;
      }else if(n==4 && num>=2){
        ans *= tmp;
        n=2;
      }
    }
    tmp = A[i];
    num=1;
  }
  if(n==4 && num>=4){
    cout << tmp*tmp << endl;
    return 0;
  }else if(n==2 && num>=2){
    ans*= tmp;
    cout << ans << endl;
    return 0;
  }
  cout << "0" << endl;
  return 0;
}