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: Time Limit Exceeded)

def check(arr,length)
  if arr.include?(length) # 2個あったら返す
    arr.delete_at(arr.index(length))
    return length
  end
  return nil
end

n = gets.chomp.to_i
sticks = gets.chomp.split(" ").map(&:to_i)

tmp = nil
side = nil
while sticks.size > 0 do

  length = sticks.max

  sticks.delete_at(sticks.index(length))


  unless side.nil?
    ret = check(sticks,length)
    unless ret.nil?
      puts ret * side
      exit
    end
  end

  side ||= check(sticks,length)

end

puts 0