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)

import java.math.BigDecimal

fun main(args: Array<String>) {
    val n = readLine()!!

    println(
            readLine()!!.split(Regex("\\s+")).map { it.toInt() }
                    .groupBy { it }
                    .mapValues { it.value.size }
                    .filterValues { it >= 2 }
                    .toList().sortedByDescending { it.first }
                    .let { l ->
                        val p1 = l.getOrElse(0, { i -> 0 to 0 })
                        val p2 = l.getOrElse(1, { i -> 0 to 0 })

                        val e1 = BigDecimal(p1.first)
                        val e2 = BigDecimal(p2.first)
                        
                        e1 * (if (p1.second >= 4) e1 else e2)
                    }
    )
}