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 Data.List

main = do
  getLine
  a <- getLine >>= return . reverse . f . sort . (map read) . words
  
  print $ g $ filter (\(x,c) -> c>=2) a

  where
    f :: [Int] -> [(Int, Int)]
    f [] = []
    f xs@(x:rest) = (x, len) : (f (drop len xs))
      where
        l = takeWhile (==x) xs
        len = length l

    g ((a,_):(b,_):_) = a*b
    g (_:[]) = 0
    g [] = 0