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

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: 'a group of people standing around a table filled with assorted electronic devices'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a sign that says pay here to park'. Find frequency of the letter 't'
Example Output:
3