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 Common Lisp (SBCL 1.1.14) (judge verdict: Time Limit Exceeded)

(defun make-rec (N A-list)
  (let* ((candidates (remove-if-not
                      (lambda (a) (>= (count a A-list) 2)) A-list))
         (sorted-candidates
           (if candidates (sort (remove-duplicates candidates :test #'=) #'>))))
    (if candidates
        (* (car sorted-candidates)
           (cadr sorted-candidates))
        0)))

(let* ((N (read))
       (A-list (loop repeat N collect (read))))
  (format t "~A~%" (make-rec N A-list)))