How big was the crowd when the opponent was essendon?
CREATE TABLE "round_1" (
  "home_team" text,
  "home_team_score" text,
  "away_team" text,
  "away_team_score" text,
  "venue" text,
  "crowd" real,
  "date" text
);
SELECT SUM("crowd") FROM "round_1" WHERE "away_team"='essendon';