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

Example Input: Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a black animal in dirt area below trees and rocks'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a silver sedan is parked in a plaza lot on a cloudy day'. Find frequency of the letter 'o'
Example Output:
3