Tell me the sum of Year for outcome of runner-up for world darts championship
CREATE TABLE "pdc_premier_event_finals_5_1_title_4_run" (
  "outcome" text,
  "year" real,
  "championship" text,
  "opponent_in_the_final" text,
  "score_l_score_in_legs_s_score_in_sets" text
);
SELECT SUM("year") FROM "pdc_premier_event_finals_5_1_title_4_run" WHERE "outcome"='runner-up' AND "championship"='world darts championship';