program main
  integer :: N
  integer :: D
  integer :: i
  integer :: out
  !
  read(*,*) N,D
  i=1
  out = N
  do
     out = out - ( 2*D + 1)
     if ( out <= 0) exit
     i = i+1
  end do
  out = i
  write(*,'(i0)') out
  stop
end program main
