what is the change in the value of the rdw of patient 63368 last measured on the last hospital visit compared to the first value measured on the last hospital visit?
select ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 63368 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'rdw' ) order by labevents.charttime desc limit 1 ) - ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 63368 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'rdw' ) order by labevents.charttime asc limit 1 )