how much was patient 808's body weight first measured since 09/2104?
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 = 808 ) ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'admit wt' and d_items.linksto = 'chartevents' ) and strftime('%y-%m',chartevents.charttime) >= '2104-09' order by chartevents.charttime asc limit 1