main :: IO ()
main = do
  instructions <- getContents
  let
    parsed = parseInstructions instructions
    location = navigate parsed
    solution1 = uncurry (*) location
  print solution1

parseInstructions :: String -> [(Int, Int)]
parseInstructions instructions =
  map parseLine (lines instructions)
  where
    parseLine line = case break (== ' ') line of
      ("forward", distance) -> (read distance, 0)
      ("down", distance)    -> (0, read distance)
      ("up", distance)      -> (0, (-read distance))
      _                     -> error "Invalid line"

navigate :: [(Int, Int)] -> (Int, Int)
navigate instructions =
  foldl1 sumTuples instructions
  where
    sumTuples :: (Int, Int) -> (Int, Int) -> (Int, Int)
    sumTuples (a, b) (c, d) = (a + c, b + d)