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

Example Input: Sentence: 'a woman with short, brown hair is looking into a circular mirror and holding a camera up to her cheek'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a couple of people that are on a baseball field'. Find frequency of the letter 'c'
Example Output: 1

Example Input: Sentence: 'a black dog next to a book titled "cesar's way."'. Find frequency of the letter 'o'
Example Output:
4