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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;cin>>N;
  vector<int> a;
  for(int i = 0; N > i; i++){
    int tmp;cin>>tmp;
    a.push_back(tmp);
  }
  sort(a.begin(),a.end(),greater<int>());
  int ret = 0;
  for(int i = 1; N > i; i++){
    if(a[i] == a[i-1]){
      if(ret != 0){
        ret *= a[i];
        break;
      }else{
        ret += a[i];
        i++;
      }
    }
  }
  cout << ret << endl;
}