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

Example Input: Sentence: 'a couple of men standing inside of a liquor store'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'a person riding a skate board on a city street'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a girl in a green shirt and hat being defended by a girl in a white dress while playing frisbee'. Find frequency of the letter 'r'
Example Output:
6