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

Example Input: Sentence: 'a cat sprawls out and sleeps on its back on a lounge chair with an acorn theme'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a cat sleeping near lots of pairs of shoes'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'an open plastic bag of hot dogs sitting on a table'. Find frequency of the letter 'f'
Example Output:
1