What series number was directed by milan cheylov and written by will dixon?
CREATE TABLE "season_1_1996_97" (
  "series_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text
);
SELECT AVG("series_num") FROM "season_1_1996_97" WHERE "directed_by"='milan cheylov' AND "written_by"='will dixon';