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 halfnize . filterDoubles $ sticks of
  [] -> 0
  (x1:x2:_) -> x1 * x2
  _ -> 0

halfnize :: [Int] -> [Int]
halfnize = filterOddIndex . sortOn (\a -> -a) 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
