what is the total dose of ns that patient 63781 was prescribed until 03/2103?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 63781 ) and prescriptions.drug = 'ns' and strftime('%y-%m',prescriptions.startdate) <= '2103-03'