How many years had a formula of Grand Prix?
CREATE TABLE "winners_of_the_circuit_des_ardennes" (
  "year" real,
  "formula" text,
  "driver" text,
  "constructor" text,
  "location" text,
  "report" text
);
SELECT COUNT("year") FROM "winners_of_the_circuit_des_ardennes" WHERE "formula"='grand prix';