program main
  implicit none

  integer :: n, d, nec

  read *, n, d

  if ( mod ( n, 2 * d + 1 ) == 0 ) then
     nec = n / ( 2.0 * d + 1.0 )
  else
     nec = n / ( 2.0 * d + 1.0 ) + 1
  end if
  
  print *, nec

end program main

