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 Kotlin (1.3.71) (judge verdict: Accepted)

import java.util.*

fun main(args: Array<String>) {
  val sc = Scanner(System.`in`)
  val n = sc.nextInt()
  val a = (1..n).map { sc.nextLong() }


  val map = mutableMapOf<Long, Int>()
  for (ai in a) {
    val tmp = map.getOrElse(ai) { 0 }
    map.put(ai, tmp+1)
  }

  val mm = map.filter { it.value >= 2 }.keys.sorted().reversed()
  if (mm.size == 0) {
    println(0)
    return
  }

  val mk = mm[0]
  val mv = map.getOrElse(mk) { 0 }
  if (mv >= 4) {
    println(mk * mk)
    return
  }

  if (mm.size == 1) {
    println(0)
    return
  }

  val mkk = mm[1]
  println(mk * mkk)

}