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)

# frozen_string_literal: true

# pyでやったらなぜかサンプル以外REなったんでrubyで
# よくよく考えたらREなるの当たり前の実装だったわ

# ロジックは間違ってなかったのでお察し

N = gets.chomp.to_i
h = {}
gets.chomp.split.map(&:to_i).each do |i|
  if h.key?(i)
    h[i] += 1
  else
    h[i] = 1
  end
end

h = h.sort_by { |k, _v| k }.reverse
imtwo = []
imfour = []
ans = 0

h.each do |k, v|
  if v < 2
    next
  elsif  imfour.empty? && v >= 4
    imfour << k
  elsif imtwo.size < 2 && v >= 2
    imtwo << k
  end

  if imtwo.size == 2
    ans = imtwo.inject(:*)
    break
  elsif imfour.size == 1
    ans = imfour[0] * 2
    break
  end
end

puts ans