import           Control.Monad as M

main :: IO ()
main = do
    [n, d] <- map read . words <$> getLine
    t <- M.replicateM n $ (\[x, y] -> (x, y)) . map read . words <$> getLine
    print $ solve d t

type Pair = (Int, Int)

solve :: Int -> [Pair] -> Int
solve d ((t, k):h) = uncurry max . foldl step (t, k - d) $ h
    where step :: Pair -> Pair -> Pair
          step (accT, accK) (t, k) = (t + max accT (accK - d), k + max accK (accT - d))