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: Wrong Answer)

N = gets.to_i
AS = gets.chomp.split.map(&:to_i)

h = Hash.new { |h, k| h[k] = [] }
AS.each do |a|
  h[a] << a
end

if h.all? { |k, v| v.length < 2 }
  puts(0)
  exit
end

ns = []
h.sort_by { |k, v| -k }.each do |k, v|
  next if v.length < 2
  before = 0
  v.each do |e|
    if e == before
      ns << [e, before]
      before = 0
    else
      before = e
    end
  end
end

result = 0
a = []
while b = ns.shift
  if !a.empty? && a[0] != b[0]
    result += a[0] * b[0]
    a = []
  else
    a = b
  end
end
p result