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_13 ( total_produced INTEGER, builder’s_model VARCHAR, wheel_arrangement VARCHAR, prr_class VARCHAR )

### Question ###
How many total units were built of Model ds-4-4-660 with a b-b wheel arrangement and a PRR Class of bs6?

### Accurate SQL ###

SELECT SUM(total_produced) FROM table_name_13 WHERE wheel_arrangement = "b-b" AND prr_class = "bs6" AND builder’s_model = "ds-4-4-660"