{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE BinaryLiterals #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE NumDecimals #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections #-}

import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.ST

import Data.Char
import Data.List
import Data.Maybe

import qualified Data.ByteString.Char8 as B

import Data.Array.ST.Safe
import Data.STRef

import Debug.Trace
import Text.Printf

readInt = readLn :: IO Int
readInteger = readLn :: IO Integer
readInts = map ( fst . fromJust . B.readInt ) . B.words <$> B.getLine
readIntegers = map ( fst . fromJust . B.readInteger ) . B.words <$> B.getLine

which a b f = if f then a else b
mp [ a, b ] = ( a, b )

modifyArray a i f = writeArray a i =<< f <$> readArray a i

main = do
	[ n, d ] <- readInts
	[ ts, ks ] <- transpose <$> replicateM n readInts
	print $ solve n d ts ks

solve n d ts' ks' = runST $ do
	ts <- newListArray ( 0, n - 1 ) ts' :: ST s ( STUArray s Int Int )
	ks <- newListArray ( 0, n - 1 ) ks' :: ST s ( STUArray s Int Int )
	-- dp[ # of days ][ at Tokyo ] := maximum income
	dp <- newArray ( ( 0, 0 ), ( n, 1 ) ) minBound :: ST s ( STUArray s ( Int, Int ) Int )
	writeArray dp ( 0, 0 ) 0
	forM_ [ 0 .. n - 1 ] $ \i -> do
		forM_ [ 0, 1 ] $ \j -> do
			forM_ [ 0, 1 ] $ \k -> do
				tokyo <- readArray ts i
				kyoto <- readArray ks i
				from <- readArray dp ( i, j )
				let
					cost = if j == k then 0 else d
					gain = if k == 0 then tokyo else kyoto
				modifyArray dp ( i + 1, k ) ( max $ from - cost + gain )
	res1 <- readArray dp ( n, 0 )
	res2 <- readArray dp ( n, 1 )
	return $ max res1 res2