Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
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?

### Inputs:
table_name: table, columns: ['Year', 'Winner', 'Country', 'Score', 'To par', '1st prize ( $ )']

### Response:
SELECT COUNT 1st prize ( $ ) FROM table WHERE Country = united states AND Score < 271 AND Year > 1964 AND Winner = bobby mitchell