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 group of giraffes walk together by a hill'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a young boy who is laying down and has something in his hand'. Find frequency of the letter 'e'
Example Output:
1