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)

n = gets.to_i
an = gets.strip.split.map(&:to_i).sort.reverse

memo = {}
two = []
four = []
an.each do |a|
  if memo[a].nil?
    memo[a] = 1
  else
    memo[a] += 1
  end
  if memo[a] == 2
    two << a
  end
  if memo[a] == 4
    four << a
  end
end

if four.size > 0
  if two.size >= 2
    puts [four[0] * four[0], two[0] * two[1]].max
  else
    puts four[0] * four[0]
  end
else
  if two.size >= 2
    puts two[0] * two[1]
  else
    puts 0
  end
end