What is largest series number for the episode that aired February 16, 1957?
CREATE TABLE "season_2_1956_57" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "director" text,
  "writer_s" text,
  "airdate" text
);
SELECT MAX("series_num") FROM "season_2_1956_57" WHERE "airdate"='February 16, 1957';