What is the smallest number of seats in a retired vehicle that was started in service in 1981?
CREATE TABLE "historical_fleet" (
  "make" text,
  "year_placed_in_service" text,
  "quantity" real,
  "number_of_seats" real,
  "wheelchair_accessible" text,
  "current_status" text
);
SELECT MIN("number_of_seats") FROM "historical_fleet" WHERE "current_status"='retired' AND "year_placed_in_service"='1981';