1a.hs

import Data.List
import Data.Sequence (Seq (..), fromList)
import Prelude

main = do
  input <- getContents
  case find2020 $ fromList $ sort $ map read $ lines input of
    Just x -> putStr $ show x
    Nothing -> putStr "Not Found"

find2020 :: Seq Int -> Maybe Int
find2020 Empty = Nothing
find2020 xs
  | sum == 2020 = Just $ head * last
  | sum < 2020 = find2020 tail
  | otherwise = find2020 init
  where
    sum = head + last
    head :<| tail = xs
    init :|> last = xs