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)

# https://atcoder.jp/contests/arc081/submissions/9271251
n = gets.to_i
nums = gets.split.map(&:to_i)
edges = []
nums_cnt = Hash.new(0)
nums.each { |i| nums_cnt[i] += 1 }
nums_cnt.keys.sort.reverse.each do |k|
  v = nums_cnt[k]
  next if v < 2
  edges << k
  edges << k if v >= 4
  break if edges.size >= 2
end
puts edges.size >= 2 ? edges[0] * edges[1] : 0