let () =
  let l, k = read_line () |> Str.split (Str.regexp_string " ") |>
             List.map int_of_string |>
             fun lst -> (List.nth lst 0, List.nth lst 1)
  in
  let kaisu = (l - 1) / k + 1 in
  (kaisu - 1) / 2 * k |> string_of_int |> print_endline