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 person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a computer desk is against the wall of small living room'. Find frequency of the letter 'o'
Example Output:
4