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 read () = Scanf.scanf "%d " (fun i -> i)

let () =
  let n = read () in
  let a = Array.init n (fun _ -> read ()) in
  Array.fast_sort (fun x y -> y - x) a;
  let rec doit i pre =
    if i = n then 0
    else if a.(i-1) = a.(i) then begin
      if pre <> (-1) then pre * a.(i)
      else doit (i + 2) a.(i)
    end else doit (i + 1) pre in
  doit 1 (-1) |> Printf.printf "%d\n"