what's the total dose of gentamicin sulfate that patient 40435 has been prescribed until 11/2105?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 40435 ) and prescriptions.drug = 'gentamicin sulfate' and strftime('%y-%m',prescriptions.startdate) <= '2105-11'