How many episodes were written only by William N. Fordes?
CREATE TABLE "table1_12159115_3" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT COUNT("series_num") FROM "table1_12159115_3" WHERE "written_by"='William N. Fordes';