toInt :: (String -> Int)
toInt x = read x :: Int

getInts = do
    x <- getLine
    let y = (map$toInt)$words x
    return y

main = do
    x <- getInts
    let n = x !! 0 :: Int
    let d = x !! 1 :: Int
    let scope = 2 * d + 1 :: Int

    if (n `mod` scope /= 0)
        then print ((n `div` scope) + 1)
        else print (n `div` scope)