how much is the difference between patient 28959's arterial bp mean second measured on the first intensive care unit visit compared to the value first measured on the first intensive care unit visit?
select ( 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 = 28959 ) and icustays.outtime is not null order by icustays.intime asc limit 1 ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'arterial bp mean' and d_items.linksto = 'chartevents' ) order by chartevents.charttime asc limit 1 offset 1 ) - ( 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 = 28959 ) and icustays.outtime is not null order by icustays.intime asc limit 1 ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'arterial bp mean' and d_items.linksto = 'chartevents' ) order by chartevents.charttime asc limit 1 )