class Calc0063
  def initialize(args)
    args = args.map { |l| l.chomp.split(/\s+/) }
    @l, @k = args.shift.map(&:to_i)
  end

  def run
    if @l % (@k * 2) == 0
      (@l / (@k * 2) - 1) * @k
    else
      @l / (@k * 2) * @k
    end
  end
end

puts Calc0063.new(STDIN.readlines).run if __FILE__ == $0