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

Example Input: Sentence: 'a small dog sits in the rear window of a car'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a room with a big screen television and a couch'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'manual for keyboard and mouse with keyboard and mouse in background'. Find frequency of the letter 'r'
Example Output:
4