how many patients in 2105 were prescribed epidural bag within 2 months after having undergone non-invasive mech vent?
select count( distinct t1.subject_id ) from ( select admissions.subject_id, procedures_icd.charttime from procedures_icd join admissions on procedures_icd.hadm_id = admissions.hadm_id where procedures_icd.icd9_code = ( select d_icd_procedures.icd9_code from d_icd_procedures where d_icd_procedures.short_title = 'non-invasive mech vent' ) and strftime('%y',procedures_icd.charttime) = '2105' ) as t1 join ( select admissions.subject_id, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where prescriptions.drug = 'epidural bag' and strftime('%y',prescriptions.startdate) = '2105' ) as t2 on t1.subject_id = t2.subject_id where t1.charttime < t2.startdate and datetime(t2.startdate) between datetime(t1.charttime) and datetime(t1.charttime,'+2 month')