text: Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
  ### Context:
  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?
  ### Answer:
  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"