What is the total number of 1st prize ($) that has a United States country with a score lower than 271 and in a year after 1964 with a winner of Bobby Mitchell?
CREATE TABLE "winners" (
  "year" real,
  "winner" text,
  "country" text,
  "score" real,
  "to_par" text,
  "1st_prize" real
);
SELECT COUNT("1st_prize") FROM "winners" WHERE "country"='united states' AND "score"<271 AND "year">1964 AND "winner"='bobby mitchell';