tell me the variation in the potassium value of patient 12775 measured at 2105-12-31 03:04:00 compared to the value measured at 2105-12-30 03:06:00?
select ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 12775 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'potassium' ) and labevents.charttime = '2105-12-31 03:04:00' ) - ( select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 12775 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'potassium' ) and labevents.charttime = '2105-12-30 03:06:00' )