What is the total number of ratings among the 18 to 49 group that was aired on January 7, 2009?
CREATE TABLE "weekly_ratings" (
  "episode" text,
  "air_date" text,
  "timeslot_est" text,
  "season" real,
  "rating" real,
  "share" real,
  "18_49" real,
  "viewers_m" real,
  "rank_num" text
);
SELECT COUNT("18_49") FROM "weekly_ratings" WHERE "air_date"='january 7, 2009';