What is the sum of the round of the player who plays linebacker and has a pick # greater than 251?
CREATE TABLE "nfl_draft" (
  "round" real,
  "pick_num" real,
  "player" text,
  "position" text,
  "college" text
);
SELECT SUM("round") FROM "nfl_draft" WHERE "position"='linebacker' AND "pick_num">251;