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 Haskell (GHC 7.10.3) (judge verdict: Wrong Answer)

import Control.Monad
import Control.Applicative
import Data.List
import Data.Array

strToInt s = (read :: String -> Int)
           
           
main = do

  n <- readLn :: IO Int
  xn <- map read . words <$> getLine
  
  let an = reverse $ sub  $ sort xn
  
  if (length an) > 1 then print ( (an !! 0) * (an !! 1) )
                     else print 0


sub :: [Int] -> [Int]
sub [] = []
sub (x:[]) = []
sub (x:xn) = if (x == (head xn))
             then [x] ++ sub (tail xn)
             else sub (tail xn)