tell me the average roll for the featherston area and integrated authority.
CREATE TABLE "south_wairarapa_district" (
  "name" text,
  "years" text,
  "gender" text,
  "area" text,
  "authority" text,
  "decile" real,
  "roll" real
);
SELECT AVG("roll") FROM "south_wairarapa_district" WHERE "area"='featherston' AND "authority"='integrated';