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

Example Input: Sentence: 'a white toilet sitting next to a bathroom sink'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a display case filled with lots of different clocks'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a man giving a thumbs up behind a computer screen'. Find frequency of the letter 'c'
Example Output:
2