How many assists were in the k-league competition, which has more than 20 total Gs, the pohang steelers team, and chunnam dragons as the opponent?
CREATE TABLE "members" (
  "name" text,
  "date" text,
  "goals" real,
  "assists" real,
  "team" text,
  "venue" text,
  "opponent" text,
  "competition" text,
  "total_gs" real,
  "total_as" real
);
SELECT "assists" FROM "members" WHERE "competition"='k-league' AND "total_gs">20 AND "team"='pohang steelers' AND "opponent"='chunnam dragons';