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)

def main()
  n = gets.chomp.to_i
  x = gets.chomp.split(' ').map(&:to_i)
  dic = Hash.new(0)
  for i in 0..n-1 do
    dic[x[i]] = dic[x[i]] + 1
  end
  target1 = dic.select{|k,v| v >=2}.keys.sort.reverse
  target2 = dic.select{|k,v| v >=4}.keys.sort.reverse
  ans1 = 0
  if target2.size != 0
    ans1 = target2[0]*target2[0]
  end
  if target1.size < 2
    puts [0,ans1].max
  else
    puts [target1[0]*target1[1],ans1].max
  end
end

main()