What is the episode number in the series for "graduation" that was directed by Don Corvan?
CREATE TABLE "table1_20967430_4" (
  "ep" real,
  "season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "prod_code" text
);
SELECT MIN("ep") FROM "table1_20967430_4" WHERE "directed_by"='Don Corvan' AND "title"='\"Graduation\"';