defmodule Main do
  def main do
    [l, k] = IO.gets("")  |> String.trim
                          |> String.split
                          |> Enum.map(&String.to_integer/1)
    case {div(l, 2 * k) * k, rem(l, 2 * k)} do
      {a, 0} -> a - k
      {a, _} -> a
    end |> Integer.to_string |> IO.puts
  end
end