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: 'several zebras  and a giraffe in an enclosure'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a carousel cake with pink and blue frosting'. Find frequency of the letter 't'
Example Output:
2