solve :: [Integer] -> Integer -> Integer -> Integer -> Integer
solve [] _ _ acc = acc
solve (x:xs) end_of_water t acc
  | end_of_water <= x = solve xs (x+t) t (acc+t)
  | otherwise          = solve xs (x + t) t (acc - (end_of_water - x) + t)

main = do
     nt <- words <$> getLine
     let n = read $ (nt !! 0) :: Integer
     let t = read $ (nt !! 1) :: Integer
     -- putStrLn $ show $ n
     -- putStrLn $ show $ t

     ts_str <- words <$> getLine
     let ts = map (\s -> read s :: Integer) ts_str
     let ans = solve ts 0 t 0
     putStrLn $ show $ ans
     return ()
