for the last time, when they came to the hospital first time how much etomidate was prescribed to patient 18677?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18677 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and prescriptions.drug = 'etomidate' order by prescriptions.startdate desc limit 1