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

Example Input: Sentence: 'three people are gathered around sitting on a park bench'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a small white dog looking out a car window'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a tennis player in an indoor tennis court wears black and holds his racket'. Find frequency of the letter 'r'
Example Output:
5