what is the monthly average white blood cells level of patient 3677 until 2 years ago?
select avg(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3677 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'white blood cells' ) and datetime(labevents.charttime) <= datetime(current_time,'-2 year') group by strftime('%y-%m',labevents.charttime)