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(arg: Array<String>) {
    val N = nextInt()
    val A = listOfLong()
    val M = mutableMapOf<Long, Int>()
    A.sortedDescending().forEach { a ->
        M[a] = M.getOrElse(a, { 0 }) + 1
    }
    val F = M.filter { it.value >= 2 }.toList()
    val ans = when (F.size) {
        0 -> 0L
        1 -> if (F[0].second >= 4) F[0].first * F[0].first else 0L
        else -> F[0].first * F[if (F[0].second >= 4) 0 else 1].first
    }
    println(ans)
}

fun next() = readLine()!!
fun nextInt() = next().toInt()
fun listOfString() = next().split(" ")
fun listOfLong() = listOfString().map { java.lang.Long.parseLong(it) }