tell me the first value of po2 in a lab test of patient 3403 in 07/2105?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3403 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'po2' ) and strftime('%y-%m',labevents.charttime) = '2105-07' order by labevents.charttime asc limit 1