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;
#define rep(i, n) for(int i = 0; i < (n); i++)
typedef long long int ll;
typedef pair<int, int> P;
const ll MOD = 1e9+7, INF = 1e9;

int main()
{
  int n;
  cin >> n;
  vector<int> v(n);
  rep(i, n){
    cin >> v[i];
  }
  sort(v.begin(), v.end());
  reverse(v.begin(), v.end());
  int a[100000] = {0,0};
  int k = 0;
  for(int i = 0; i < n-1; i++){
    if(v[i] == v[i+1]){
      a[k++] = v[i];
      i++;
    }
  }
  cout << a[0]*a[1] << endl;
  system("pause");
}