how many people were diagnosed with choledochlith nos w obst and did not visit the hospital within 2 months this year?
select ( select count( distinct t1.subject_id ) from ( select admissions.subject_id, diagnoses_icd.charttime from diagnoses_icd join admissions on diagnoses_icd.hadm_id = admissions.hadm_id where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'choledochlith nos w obst' ) and datetime(diagnoses_icd.charttime,'start of year') = datetime(current_time,'start of year','-0 year') ) as t1 ) - ( select count( distinct t2.subject_id ) from ( select admissions.subject_id, diagnoses_icd.charttime from diagnoses_icd join admissions on diagnoses_icd.hadm_id = admissions.hadm_id where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'choledochlith nos w obst' ) and datetime(diagnoses_icd.charttime,'start of year') = datetime(current_time,'start of year','-0 year') ) as t2 join admissions on t2.subject_id = admissions.subject_id where t2.charttime < admissions.admittime and datetime(admissions.admittime,'start of year') = datetime(current_time,'start of year','-0 year') and datetime(admissions.admittime) between datetime(t2.charttime) and datetime(t2.charttime,'+2 month') )