In this task, you need to count the number of times the given letter appears in the given sentence.

Example Input: Sentence: 'a man holding a frisbee high into the air'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a street scene looking at the back of the stop sign'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'an umpire behind a batter hitting a ball at a baseball game'. Find frequency of the letter 't'
Example Output:
5