มีกี่แผนกที่นำโดยหัวหน้าที่ไม่ได้กล่าวถึง?
How many departments are led by heads who are not mentioned?
management_df = pd.DataFrame(columns=['department_id']); department_df = pd.DataFrame(columns=['department_id'])
department_df['department_id'].isin(management_df['department_id']).value_counts()[False]