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 OCaml (4.02.3) (judge verdict: Accepted)

let mkls n =
  let rec f n' ls =
    if n' = 0 then List.fast_sort (fun i j -> j - i) ls
    else
      f (pred n') (Scanf.scanf "%d " (fun x -> x) :: ls)
  in
  f n []

let solve ls =
  let rec f ed = function
    | [] | [_] -> 0
    | h :: (h' :: t as t') ->
      if h = h' && ed > 0 then h * ed
      else if h = h' && ed <= 0 then f h t
      else f ed t'
  in
  f 0 ls

let () = Scanf.scanf "%d\n" mkls |> solve |> Printf.printf "%d\n"