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: Accepted)

import Control.Applicative( (<$>))
import Data.ByteString.Char8( readInt, getLine, words	)
import qualified Data.ByteString.Char8 as C8
import Data.Maybe( fromJust)
import Data.List( sortBy)

main = do
    let
	readBS2Int = fst . fromJust . readInt
	readLine2Ints = map readBS2Int . C8.words <$> C8.getLine

    n_ <- readLn::IO Int
    as_ <- readLine2Ints
    let
	desc = sortBy (flip compare) as_

	collect::Int->Int->[Int]->Integer
	collect 0 0 []	= 0
	collect 0 0 [a]	= 0
	collect 0 0 (a1:tl@(a2:as))
	  | a1==a2	    = collect a1 0 as
	  | otherwise	    = collect 0 0 tl
	collect l 0 [] = 0
	collect l 0 [a] = 0
	collect l 0 (a1:tl@(a2:as))
	  | a1==a2	    = fromIntegral l* fromIntegral a2
	  | otherwise	    = collect l 0 tl
    
    print $ collect 0 0 desc