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)

module CharMap = Map.Make (Char)

let alphabet =
  [ 'a'; 'b'; 'c'; 'd'; 'e'; 'f'; 'g'; 'h'; 'i'; 'j'; 'k'; 'l'; 'm';
    'n'; 'o'; 'p'; 'q'; 'r'; 's'; 't'; 'u'; 'v'; 'w'; 'x'; 'y'; 'z' ]

let () =
  let a = read_line () in
  let trie = Array.make (String.length a + 1) CharMap.empty in
  for i = String.length a - 1 downto 0 do
    trie.(i) <- CharMap.add a.[i] (i + 1) trie.(i + 1)
  done;
  let dp = Array.make (String.length a + 1) (0, []) in
  for i = String.length a downto 0 do
    dp.(i) <- List.fold_right (fun c -> min @@
      match dp.(CharMap.find c trie.(i)) with
      | exception Not_found -> (1, [c])
      | (n, s) -> (1 + n, c :: s)) alphabet (max_int, [])
  done;
  List.iter print_char (snd dp.(0));
  print_newline ()