for the last time, how much potassium chloride was prescribed during this month to patient 15986?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15986 ) and prescriptions.drug = 'potassium chloride' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month') order by prescriptions.startdate desc limit 1