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

Example Input: Sentence: 'a photo of a toilet which is not clean'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a plane with stairs next to it sitting in a large lot'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a woman strains pasta out of a pot on the stove'. Find frequency of the letter 'a'
Example Output:
6