What's the average attendance of the leagues in the season of 2013?
CREATE TABLE "top_10_in_total_attendance" (
  "league" text,
  "sport" text,
  "country" text,
  "season" text,
  "games" real,
  "average_attendance" real,
  "total_attendance" real
);
SELECT MIN("average_attendance") FROM "top_10_in_total_attendance" WHERE "season"='2013';