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 qualified Data.Map as M

main = do
  getLine
  as <- map read . words <$> getLine
  print . h . M.toDescList $ f M.empty as

f m [] = M.filter (> 0) $ M.map g m
f m (a:as) = M.insertWith (const succ) a 1 (f m as)

g a
  | a >= 4 = 2
  | a >= 2 = 1
  | otherwise = 0

h ((k, v):(k', _):_)
  | v == 2 = k * k
  | otherwise = k * k'
h _ = 0