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

Example Input: Sentence: 'a man serving himself pizza in a wooden booth'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a chair and umbrella on a sandy beach'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'an asian man in a dress shirt and tie posing for a picture'. Find frequency of the letter 't'
Example Output:
3