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

Example Input: Sentence: 'a large bathroom with marble floor, counter, and bath'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'party cakes in plastic containers, with a cake with a plastic santa claus and reindeer on top'. Find frequency of the letter 's'
Example Output: 6

Example Input: Sentence: 'a woman standing on a piece of concrete with her jacket in her hand near her back'. Find frequency of the letter 'r'
Example Output:
5