What is the lowest quantity of the 1b n2 type, which was retired in 1907-12?
CREATE TABLE "locomotives_and_wagons" (
  "class" text,
  "quantity" real,
  "type" text,
  "delivered" text,
  "k_b_sts_b_class" text,
  "retired" text
);
SELECT MIN("quantity") FROM "locomotives_and_wagons" WHERE "type"='1b n2' AND "retired"='1907-12';