fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val l = readInt ()
        val k = readInt ()

        fun findAns remain =
            if remain <= 2 * k then 0
            else k + findAns (remain - 2 * k)

        val ans = findAns l
    in
        print (Int.toString ans)
    end