How many total units were built of Model ds-4-4-660 with a b-b wheel arrangement and a PRR Class of bs6?
CREATE TABLE "baldwin_locomotive_works" (
  "prr_class" text,
  "builder_s_model" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "service" text,
  "power_output" text
);
SELECT SUM("total_produced") FROM "baldwin_locomotive_works" WHERE "wheel_arrangement"='b-b' AND "prr_class"='bs6' AND "builder_s_model"='ds-4-4-660';