until 08/24/2105, what was the first value of arterial bp [diastolic] for patient 23938?
select chartevents.valuenum from chartevents where chartevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 23938 ) ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'arterial bp [diastolic]' and d_items.linksto = 'chartevents' ) and strftime('%y-%m-%d',chartevents.charttime) <= '2105-08-24' order by chartevents.charttime asc limit 1