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

Example Input: Sentence: 'a man paddle boarding by a boat dock in a city'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a couple of buses parked on the side of the street'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a table that has been set with two pizza pies'. Find frequency of the letter 'e'
Example Output:
5