what's the total furosemide dose that patient 3929 has been prescribed in 05/2105?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3929 ) and prescriptions.drug = 'furosemide' and strftime('%y-%m',prescriptions.startdate) = '2105-05'