What is the year that the Roughriders left the conference?
CREATE TABLE "former_members" (
  "school" text,
  "city" text,
  "mascot" text,
  "county" text,
  "year_joined" real,
  "previous_conference" text,
  "year_left" real,
  "conference_joined" text
);
SELECT MAX("year_left") FROM "former_members" WHERE "mascot"='roughriders';