how many patients was diagnosed with hypopotassemia during the same hospital encounter after being diagnosed with left bb block nec?
select count( distinct t1.subject_id ) from ( select admissions.subject_id, diagnoses_icd.charttime, admissions.hadm_id 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 = 'hypopotassemia' ) ) as t1 join ( select admissions.subject_id, diagnoses_icd.charttime, admissions.hadm_id 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 = 'left bb block nec' ) ) as t2 on t1.subject_id = t2.subject_id where t1.charttime < t2.charttime and t1.hadm_id = t2.hadm_id