how many times adefovir dipivoxil has been prescribed to patient 18159 on their last hospital encounter?
select count(*) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18159 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'adefovir dipivoxil'