what is the difference in red blood cells levels of patient 42829 second measured on the current hospital visit compared to the value first measured on the current hospital visit?
select ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 42829 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'red blood cells' ) order by labevents.charttime asc limit 1 offset 1 ) - ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 42829 and admissions.dischtime is null ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'red blood cells' ) order by labevents.charttime asc limit 1 )