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

Example Input: Sentence: 'there is a sandwich with a lot of meat cut in half'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a snow filled mountain with a guy with ski's'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a man sitting on top of a couch next to a fire place'. Find frequency of the letter 'n'
Example Output:
4