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
M = gets.split.map(&:to_i).group_by(&:itself)
    .map{|k,v| [k,v.size]}
    .select{|k,z| z >= 2 }
    .max_by(2){|k,z| k }

if M.empty?
  puts -1
elsif M.size == 1 
  if M[0][1] >= 4
    puts M[0][0] ** 2
  else
    puts -1
  end
else
  if M[0][1] >= 4
    puts M[0][0] ** 2
  else
    puts M[0][0] * M[1][0]
  end
end