Question: 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?
SQL: SELECT COUNT("1st prize ( $ )") FROM "2-12919860-2" WHERE "Country" = 'united states' AND "Score" < 271.0 AND "Year" > 1964.0 AND "Winner" = 'bobby mitchell';