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
import Data.Ord

takePairs []  = []
takePairs [_] = []
takePairs (a:b:xs)
  | a == b = a : takePairs xs
  | otherwise = takePairs (b:xs)

main :: IO ()
main = do
    n <- readLn :: IO Int
    a <- map read . words <$> getLine :: IO [Int]
    let a' = takePairs . concat
           . filter ((2<=) . length) . group . sortOn Down $ a
    print $ if length a' < 2
               then 0 else product $ take 2 a'