open System
open System.Collections.Generic

[<AutoOpen>]
module Cin =
    let read f = stdin.ReadLine() |> f
    let reada f = stdin.ReadLine().Split() |> Array.map f
    let readChars() = read string |> Seq.toArray
    let readInts() = readChars() |> Array.map (fun x -> Convert.ToInt32(x.ToString()))

[<AutoOpen>]
module Cout =
    let writer = new IO.StreamWriter(new IO.BufferedStream(Console.OpenStandardOutput()))
    let print (s: string) = writer.Write s
    let println (s: string) = writer.WriteLine s
    let inline puts (s: ^a) = string s |> println

module Util =
    let strRev (s: string): string =
        s
        |> Seq.rev
        |> Seq.map string
        |> String.concat ""

    let inline roundup (x: ^a) (y: ^a): ^a =
        let one = LanguagePrimitives.GenericOne
        (x + y - one) / y

    let inline sameParity (x: ^a) (y: ^a): bool =
        let one = LanguagePrimitives.GenericOne
        (x &&& one) = (y &&& one)

let solve() =
    let [| l; k |] = reada int
    let cnt = Util.roundup l (k * 2)
    let ans = (cnt - 1) * k
    puts ans
    ()

[<EntryPoint>]
let main _ =
    try
        solve()
    with e -> printfn "%s" (e.ToString())
    writer.Close()
    0 // return an integer exit code