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: Accepted)

fun main(args : Array<String>) {
    val n = readLine()!!.toInt()
    val aa = readLine()!!.split(" ").map { it.toLong() }.groupBy { it }.filter { it.value.size >= 2 }

    if (aa.isEmpty()) {
        println(0)
        return
    }

    if (aa.size == 1) {
        val a = aa.entries.first()
        if (a.value.size < 4) {
            println(0)
        } else {
            println(a.key * a.key)
        }
        return
    }

    val sorted = aa.entries.sortedByDescending { it.key }
    if (sorted[0].value.size >= 4) {
        println(sorted[0].key * sorted[0].key)
    } else {
        println(sorted[0].key * sorted[1].key)
    }
}