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

Example Input: Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a double-decker bus covered in children's illustrations driving next to a van'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'there are two bunk beds and a ladder'. Find frequency of the letter 't'
Example Output:
2