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

Example Input: Sentence: 'a person on a surfboard in the surf of a wave'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a woman in a messy kitchen with a child'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a bathroom has a large mirror and black wall'. Find frequency of the letter 'b'
Example Output:
2