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 Scala (2.11.7) (judge verdict: Accepted)

import java.util.Scanner

object Main extends App {
  val sc = new Scanner(System.in)

  val n  = sc.nextInt
  val al = Vector.fill(n)(sc.nextInt)

  val cnt   = al.zipWithIndex.groupBy(_._1)
  

  val cnt2  = cnt.filter(_._2.size >= 2)
  val cnt4  = cnt2.filter(_._2.size >= 4)
  val over2 = cnt2.keySet.toVector
  val over4 = cnt4.keySet.toVector

  val bl = (over2 ++ over4).sortWith(_ > _)

  val res =
    if(bl.length <= 1) 0L
    else bl.head.toLong * bl.tail.head.toLong

  println(res)
}