what was the first value of a lab test of potassium until 09/2104 for patient 9338?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 9338 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'potassium' ) and strftime('%y-%m',labevents.charttime) <= '2104-09' order by labevents.charttime asc limit 1