1b.hs

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

seqHead (x :<| _) = x

seqTail (_ :<| xs) = xs

main = do
  input <- getContents
  case find2020 (-1) (fromList $ sort $ map read $ lines input) of
    Just x -> putStr $ show x
    Nothing -> putStr "Not Found"

find2020 :: Int -> Seq Int -> Maybe Int
find2020 _ Empty = Nothing
find2020 (-1) (x :<| xs) = find2020 x xs
find2020 x xs =
  case findSecond x xs of
    Just y -> Just y
    Nothing -> find2020 (seqHead xs) (seqTail xs)

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