{-# LANGUAGE RecordWildCards #-}

import Control.Monad.State.Lazy
import Data.List

data Question = Question { days :: Int, moveCost :: Int, tokyoIncome :: [Int], kyotoIncome :: [Int] } deriving Show
data CalcState = CalcState { maxTokyo :: Int, maxKyoto :: Int } deriving Show

input :: IO Question
input = do
  [d, c] <- getLineData
  [tx, kx] <- fmap transpose $ sequence $ replicate d getLineData
  return Question { days = d, moveCost = c, tokyoIncome = tx, kyotoIncome = kx }
    where getLineData = fmap (map read . words) getLine

process :: Question -> Int
process Question {..} = let CalcState {..} = execState (process' 1) $ CalcState { maxTokyo = 0, maxKyoto = -moveCost }
                         in max maxTokyo maxKyoto
  where
    process' :: Int -> State CalcState ()
    process' d
      | d > days = return ()
      | otherwise = do
          modify $ \CalcState {..} ->
            let (maxTokyo', maxKyoto') =
                  (tokyoIncome !! (d - 1) + max maxTokyo (maxKyoto - moveCost),
                   kyotoIncome !! (d - 1) + max (maxTokyo - moveCost) maxKyoto)
             in CalcState { maxTokyo = maxTokyo', maxKyoto = maxKyoto' }
          process' (d + 1)

main :: IO ()
main = input >>= return . process >>= print