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: Runtime Error)

module Main where

import Data.List

main :: IO()
main = do
  getLine
  sticks <- map read . words <$> getLine
  print . solve $ sticks

solve :: [Int] -> Int
solve sticks = case map calcArea . rectCombies $ sticks of
  [] -> 0
  areas -> maximum areas

rectCombies :: [Int] -> [(Int, Int)]
rectCombies = allCombies . halfnize . filterDoubles

allCombies :: [Int] -> [(Int, Int)]
allCombies [] = []
allCombies (s:ss) = sub s ss [] where
  sub :: Int -> [Int] -> [(Int, Int)] -> [(Int, Int)]
  sub s' [] result = result
  sub s' (x:xs) result = sub s' xs ((s, x) : result)

halfnize :: [Int] -> [Int]
halfnize = filterOddIndex . sort where
  filterOddIndex :: [Int] -> [Int]
  filterOddIndex []         = []
  filterOddIndex (x1:x2:xs) = x1 : filterOddIndex xs
  filterOddIndex _          = error "要素が奇数個あるのはおかしい"

filterDoubles :: [Int] -> [Int]
filterDoubles sticks = sub sticks [] where
  sub :: [Int] -> [Int] -> [Int]
  sub [] result = result
  sub (s:ss) result
    | s `elem` ss || s `elem` result  = sub ss (s:result)
    | otherwise                       = sub ss result

calcArea :: (Int, Int) -> Int
calcArea (x, y) = x * y