calculate how many times patient 15794 had taken pe 24 fs ng during their current hospital visit.
select count(*) from inputevents_cv where inputevents_cv.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15794 and admissions.dischtime is null ) ) and inputevents_cv.itemid in ( select d_items.itemid from d_items where d_items.label = 'pe 24 fs ng' and d_items.linksto = 'inputevents_cv' )