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 (1.0.0) (judge verdict: Wrong Answer)

import java.io.BufferedReader
import java.io.InputStreamReader
import java.io.PrintWriter

fun solve() {
    val n = inf.readLine()!!.toInt()
    val a = inf.readLine()!!.split(' ').map(String::toInt)
            .groupBy { it }.map { it.key to it.value.size }.filter { it.second > 1 }.sortedBy { -it.first }
    println(if (a.size > 1) a[0].first.toLong() * a[1].first else 0)
}

fun main(args: Array<String>) {
    solve()
    ouf.close()
}

val inf = BufferedReader(InputStreamReader(System.`in`))
val ouf = PrintWriter(System.out)