how many of d5 1/2ns were prescribed to patient 18677 in total in this month?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18677 ) and prescriptions.drug = 'd5 1/2ns' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month')