for the first time, how much carbamazepine was prescribed during this month to patient 6170?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 6170 ) and prescriptions.drug = 'carbamazepine' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month') order by prescriptions.startdate asc limit 1