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: Time Limit Exceeded)

object Main {
  def main(args: Array[String]) {
    val input = io.Source.stdin.getLines().mkString("\n")
    println(kagamiMochi(input).trim())
  }

  def kagamiMochi(input: String) :String = {
    //先頭の要素を除外する
    //新しいコレクションを作る

    //要素にある数字が何個あるかカウントする
    //同じ数字が2つ以上ある時、その数字のみ残す
    //同じ数字が4つ以上ある時、
    //その組が2つ以上なければ0を出力する
    //2つずつ以上ある数字を掛け算する
    //最大値を出力する

    val map = input.split("\n")(1).split(" ").map(_.toLong).groupBy(identity).mapValues(_.size).filter(n => (n._2 >= 2))
    if(map.isEmpty) return "0"

    //4以上の値が無いときが考慮されていない
    val square = map.filter(n => n._2 >=4)
    val squareMax = if(!square.isEmpty) Some(square.map(n => (n._1 * n._1)).max) else Some(0L)

    //mapのkey値を任意の2つ抜き出して掛け算する
    val aaaa = map.keys.toSeq.combinations(2).map(n => n.product).max
    if(aaaa > squareMax.getOrElse(0L)) return aaaa.toString else squareMax.get.toString
  }


}