What is the production code for episode 31 in the series?
CREATE TABLE "table1_23937219_3" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "prod_code" real
);
SELECT MIN("prod_code") FROM "table1_23937219_3" WHERE "series_num"=31;