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

Example Input: Sentence: 'a train sits on a railroad track as a metal awning covers it'. Find frequency of the letter 'l'
Example Output: 2

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 couple of toothbrushes that are on a table'. Find frequency of the letter 'h'
Example Output:
3