Name the total number of years where runner-up and championship is us open
CREATE TABLE "mixed_doubles_finals_15_7_8" (
  "outcome" text,
  "year" real,
  "championship" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "score" text
);
SELECT COUNT("year") FROM "mixed_doubles_finals_15_7_8" WHERE "outcome"='Runner-up' AND "championship"='US Open';