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)

n = gets.to_i
as = gets.split
  .each_with_object(Hash.new(0)) { |a, h| h[a.to_i] += 1 }
  .select { |_, v| v >= 2 }

ans = 0
prev = 0

as.keys.sort.reverse.each do |k|
  v = as[k]
  s = prev * k

  if v >= 4
    s = s >= k ** 2 ? s : k ** 2
  end

  if s > ans
    ans = s
    break
  end

  prev = k
end

puts ans