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 Data.List (sortBy,group)
import Control.Arrow ((&&&))

main :: IO ()
main = do
  _ <- read <$> getLine :: IO Int
  as <- map (fromIntegral.length&&&head).group.sortBy (flip compare).map read.words <$> getLine :: IO [(Integer,Integer)]
  let ls = filter ((>1).fst) as
  if length ls > 1
    then
      if (fst.head) ls > 4
        then print ((snd.head) ls*(snd.head) ls)
        else print ((snd.head) ls*(snd.head.tail) ls)
    else print 0