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
