What is the total year of the winner who plays the halfback position with a % of points possible of 80.64%?
CREATE TABLE "winners" (
  "year" real,
  "winner" text,
  "school" text,
  "position" text,
  "points" text,
  "pct_of_points_possible" text
);
SELECT COUNT("year") FROM "winners" WHERE "position"='halfback' AND "pct_of_points_possible"='80.64%';