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)

let n = Scanf.scanf " %d" (+) 0
let a_s = Array.init n @@ fun _ -> Scanf.scanf " %d" (+) 0
let c, p, m, i = ref 0, ref a_s.(0), ref 0, ref 0
let f d = Printf.printf "%d\n" d; exit 0
let g i a = if i = 1 then f @@ !m * a else m := a
let _ = Array.(sort (fun x y -> y - x) a_s; iter (fun a -> if a = !p then (incr c; if !c >= 4 then f @@ a * a) else (if !c >= 2 then (g !i !p; incr i); c := 1; p := a)) a_s); if !c >= 2 then g !i !p; f 0