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: Wrong Answer)

open Hashtbl
let n = Scanf.scanf " %d" (+) 0
let a_s = Array.init n @@ fun _ -> Scanf.scanf " %d" (+) 0
let h = create n
let acs = Array.(iter (fun a -> replace h a @@ try find h a + 1 with _ -> 1) a_s; List.(fold (fun a c xs -> (a, c) :: xs) h [] |> filter (fun (_, c) -> c >= 2) |> sort (fun x y -> compare y x) |> of_list))
let _ = Printf.printf "%d\n" @@ if Array.length acs < 2 then 0 else fst acs.(0) * fst acs.(1)