import Data.List

main = do
  [_, p] <- map read . words <$> getLine
  h <- map read . words <$> getLine
  print $ solve p h

solve :: Int -> [Int] -> Int
solve p a = uncurry min . foldl' f (0, p) $ zip a (tail a)
  where
    f :: (Int, Int) -> (Int, Int) -> (Int, Int)
    f (dpR, dpL) (h0, h1) = (v0, v1)
      where
        r = max 0 (h1 - h0)
        l = max 0 (h0 - h1)
        v0 = minimum [dpR + r, dpR + p, dpL + p]
        v1 = minimum [dpL + l, dpL + p, dpR + p]