
import qualified Data.Vector.Unboxed as U
import qualified Data.Vector as V
import qualified Data.ByteString.Char8 as C
import qualified Data.Array.IO as A
import Debug.Trace
import Data.List
import Control.Monad
import Data.Ord


r' :: IO [Int]
r'  =   unfoldr    (C.readInt . C.dropWhile (==' ')) <$> C.getLine


main :: IO ()
main = do
    [n,d] <- r'
    let f = fromIntegral $ d*2 + 1
    print $ ceiling $ (fromIntegral n) / f

