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

Example Input: Sentence: 'a bunch of people are standing out in a field a red one has their arms out as a white ball is in the air'. Find frequency of the letter 'e'
Example Output: 9

Example Input: Sentence: 'baseball player reaching back as he's about to throw the ball'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a car and a pick-up parked in front of a cafe on a one way street'. Find frequency of the letter 'c'
Example Output:
3