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 Ruby (2.3.3) (judge verdict: Accepted)

def compress_array_to_hash(array)
  hash = {}
  array.map { |e|
    if hash[e].nil?
      hash[e] = 1
    else
      hash[e] += 1
    end
  }
  hash
end

n = gets
as = compress_array_to_hash(gets.split.map(&:to_i)).sort_by { |key, value| key }.reverse
use = []
until use.length == 2 || as.empty?
  temp = as.shift
  if temp[1] >= 4
    use += [temp[0], temp[0]]
  elsif temp[1] >= 2
    use << temp[0]
  end
end
use += [0, 0]
puts use[0] * use[1]