{-# LANGUAGE BangPatterns #-}

import           Control.Monad.State
import           Data.Char
import           Data.Coerce
import           Data.IORef
import           Data.Word
import qualified Data.ByteString.Char8             as BSC8
import qualified Data.Vector.Fusion.Stream.Monadic as VFSM
import qualified Data.Vector.Unboxed               as VU

main :: IO ()
main = do
  [n, d] <- map (read :: String -> Int) . words <$> getLine
  [t1, k1] <- map (read :: String -> Int) . words <$> getLine
  t <- newIORef t1
  k <- newIORef k1
  rep (n - 1) $ \_ -> do
    qs <- seqInput 2
    itemt <- readIORef t
    itemk <- readIORef k
    writeIORef t (max (itemt + qs VU.! 0) (itemk + qs VU.! 0 - d))
    writeIORef k (max (itemk + qs VU.! 1) (itemt + qs VU.! 1 - d))
  print =<< max <$> readIORef t <*> readIORef k

type CParser a = StateT BSC8.ByteString Maybe a

runCParser :: CParser a -> BSC8.ByteString -> Maybe (a, BSC8.ByteString)
runCParser = runStateT
{-# INLINE runCParser #-}

int :: CParser Int
int = coerce $ BSC8.readInt . BSC8.dropWhile isSpace
{-# INLINE int #-}

seqInput :: Int -> IO (VU.Vector Int)
seqInput n = VU.unfoldrN n (runCParser int) <$> BSC8.getLine
{-# INLINE seqInput #-}

stream :: Monad m => Int -> Int -> VFSM.Stream m Int
stream !l !r = VFSM.Stream step l
  where
    step x
      | x < r     = return $ VFSM.Yield x (x + 1)
      | otherwise = return $ VFSM.Done
    {-# INLINE [0] step #-}
{-# INLINE [1] stream #-}

rep :: Monad m => Int -> (Int -> m ()) -> m ()
rep n = flip VFSM.mapM_ (stream 0 n)
{-# INLINE rep #-}