program main
  implicit none
  integer i,N,D
  !read(*, *) (L(i), i = 1,N)
  read(*, *)N,D
  i = 2*D + 1
  if (mod(N,i)==0) then 
    i = N/i
  else
    i = N/i + 1
  end if


  write(*, *) i
end program main
