Which average overall has a Round of 1, and a Position of center?
CREATE TABLE "1999_new_england_patriots_draft_selectio" (
  "round" real,
  "overall" real,
  "player" text,
  "position" text,
  "college" text
);
SELECT AVG("overall") FROM "1999_new_england_patriots_draft_selectio" WHERE "round"=1 AND "position"='center';