how many days have passed from the first time patient 2518 stayed in careunit csru during the current hospital visit?
select 1 * ( strftime('%j',current_time) - strftime('%j',transfers.intime) ) from transfers where transfers.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 2518 and admissions.dischtime is null ) and transfers.careunit = 'csru' order by transfers.intime asc limit 1