import Control.Applicative ((<$>))
import Control.Monad (replicateM)

data Pos = Tokyo | Kyoto
  deriving Eq

flipPos :: Pos -> Pos
flipPos Tokyo = Kyoto
flipPos Kyoto = Tokyo

toIdx :: Pos -> Int
toIdx Tokyo = 0
toIdx Kyoto = 1

getNums :: IO [Int]
getNums = map read . words <$> getLine

{-
  dp[i][k] := i までで位置 k で何円か
  dp[i+1][k] := max( dp[i][k]+TK[k][i], dp[i][!k]+Tk[k][i]-d )
-}

main :: IO ()
main = do
  [n, d] <- getNums :: IO [Int]
  ([iniT,iniK]:tks) <- replicateM n getNums

  print $ (\[a,b] -> max a b) $ foldl (f d) [iniT,iniK-d] tks
  
f :: Int -> [Int] -> [Int] -> [Int]
f d dps tks = [dp Tokyo, dp Kyoto] where
  dp pos = (dps!!curIdx + tks!!curIdx) `max` (dps!!revIdx + tks!!curIdx - d) where
    curIdx = toIdx pos
    revIdx = toIdx $ flipPos pos