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<iostream>
#include<algorithm>
#include<stack>
#include<queue>
#include<string>
#include<functional>
#include<iomanip>
using namespace std;

#define INF 1<<21
#define DIV 1000000007

int main() {
  int N;
  int a=0, b=0;
  int A[100005];
  cin >> N;
  for (int i = 0; i < N; i++) {
	cin >> A[i];
  }
  sort(A, A + N);
  for (int i = N - 1; i >= 0; i--) {
	if (!a) {
	  if (A[i] == A[i - 1])a = A[i];
	  i--;
	}
	else if(!b){
	  if (A[i] == A[i - 1])b = A[i];
	}
  }
  cout << a*b << endl;
  return 0;
}