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 herd of zebra standing on top  of a grass covered field'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a person in the distance looks at a frisbee flying overhead'. Find frequency of the letter 'c'
Example Output:
1