[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reads f = stdin.ReadLine().Split() |> Array.toList |> List.map f

[<EntryPoint>]
let main _ =
    let [n;d] = reads int
    let a = (n + 2 * d) / (2 * d + 1)
    printfn "%i" a
    0