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

Example Input: Sentence: 'a man is taking a picture of a man and woman'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a girl playing with a frisbee while people watch'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a green clock that is sitting above a bench'. Find frequency of the letter 'c'
Example Output:
3