def get_users_count(period):
    if period == 'day':
        today = datetime.now().strftime('%Y-%m-%d')
        return db.users.count_documents({"reg_date": today})
    elif period == 'month':
        this_month = datetime.now().strftime('%Y-%m-')
        all_days = []
        for day in range(1,32):
            all_days.append(f'{this_month}{day}')
        return db.users.count_documents({"reg_date": {'$in': all_days}})
    elif period == 'total':
        return db.users.count_documents({'user_id': {'$exists':True}})
    else:
        return -1