
# describe Abc134B do
#   context 'test' do
#     it 'sample' do
#       expect(Abc134B.calc(6, 2)).to be(2)
#       expect(Abc134B.calc(14, 3)).to be(2)
#       expect(Abc134B.calc(20, 4)).to be(3)
#     end
#   end
# end

class Abc134B
  def self.input()
    n, d = STDIN.gets.split(/\s/).map(&:to_i)
    [n, d]
  end

  def self.output(result)
    puts result
  end

  def self.calc(n, d)
    (n.to_f / (2 * d + 1)).ceil
  end

  def self.main()
    n, d = input()
    result = calc(n, d)
    output(result)
  end
end

if __FILE__ == $0
  Abc134B.main
  exit
end