
foo :: Int -> Int -> Int
foo n d =
  let
    r = 2 * d + 1
    k = n `mod` r
    a = n `div` r
  in if k == 0 then a else a + 1


main = do
  ints <- map read .words <$> getLine :: IO [Int]
  print $ foo (head ints) (last ints)
