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

Example Input: Sentence: 'this yellow fire hydrant sits in a grass field'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a tennis player after hitting a shot back to the other side of the court'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a boy wearing a pink dress shirt, and a beige tie'. Find frequency of the letter 'e'
Example Output:
5