For how many years combined were Frank Gifford and Dan Dierdorf color commentators?
CREATE TABLE "1990s" (
  "year" real,
  "network" text,
  "play_by_play" text,
  "color_commentator_s" text,
  "sideline_reporter_s" text
);
SELECT COUNT("year") FROM "1990s" WHERE "color_commentator_s"='frank gifford and dan dierdorf';