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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
  $n = trim(fgets(STDIN));
  $a = explode(" ",trim(fgets(STDIN)));
  $c = 0;
  $hen = array_fill(1,$n,0);
  $len = [];
  for($i=0; $i<$n; $i++){
      $hen[$a[$i]]++;
      if($hen[$a[$i]] == 2){
          $c++;
          $len[] = $a[$i];
          $hen[$a[$i]] = 0;
      }
  }
//   print_r($len);
  rsort($len);
  if($c > 1){
      echo $len[0]*$len[1];
  }else{
      echo "0";
  }