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

Example Input: Sentence: 'a black bear sitting in the grass taking a rest'. Find frequency of the letter 'a'
Example Output: 6

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

Example Input: Sentence: 'a desk witgmh a telephone, laptop, cell phone and a book on it'. Find frequency of the letter 'a'
Example Output:
5