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

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 girl that has a kite in the air'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a pitcher on the mound prepares to throw the base ball'. Find frequency of the letter 'p'
Example Output:
3