Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_81 (series__number INTEGER, directed_by VARCHAR, written_by VARCHAR)

### Question ###
What series number was directed by milan cheylov and written by will dixon?

### Accurate SQL ###

SELECT AVG(series__number) FROM table_name_81 WHERE directed_by = "milan cheylov" AND written_by = "will dixon"