-- AtCoder Beginner Contest 134 B
import Control.Monad
import Control.Applicative
import Data.List

toInt :: String -> Int
toInt x = read x :: Int
 
getInts = do
  x <- getLine
  let y = (map$toInt)$words x
  return y

abc134b :: [Int] -> Int
abc134b (n:(d:rm)) = ceiling $ (fromIntegral n) / (fromIntegral (2*d + 1))

main :: IO()
main = do
  nd <- getInts
  let ans = abc134b nd
  print ans