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

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: 'some food on plates next to each other on a table'. Find frequency of the letter 'e'
Example Output: 6

Example Input: Sentence: 'a sandwiche and brocolli is sitting on a yellow plate'. Find frequency of the letter 'c'
Example Output:
2