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


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

        val ans =
            if l mod (2 * k) = 0 then (l div (2 * k) - 1) * k
            else (l div (2 * k)) * k
    in
        print (Int.toString ans)
    end