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)

require 'pp'

# Nと空白区切の入力値を数値の配列で返却する
def gets_n_and_i_list()
  return gets.chomp.to_i, gets.chomp.split(" ").map(&:to_i)
end

N, a_list = gets_n_and_i_list

edge_hash = a_list.group_by(&:itself).map{|k,v| [k, v.count] }.to_h

edge_hash = edge_hash.select do |k, v|
  v >= 2
end

if edge_hash.empty?
  puts 0
  exit
end

e1 = 0
edge_hash.keys.sort.reverse_each do |k|
  v = edge_hash[k]
  if v >= 4 && e1.zero?
    puts k * k
    exit
  elsif !e1.zero?
    puts e1 * k
    exit
  else
    e1 = k
  end
end