What is the least value for average attendance when average on previous season is +644?
CREATE TABLE "average_crowd_sizes" (
  "season" text,
  "ave_attendance" real,
  "ave_on_prev_season" text,
  "competition" text,
  "occupier" text
);
SELECT MIN("ave_attendance") FROM "average_crowd_sizes" WHERE "ave_on_prev_season"='+644';