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

Example Input: Sentence: 'a man hits a tennis ball with a racket on the court'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'the counter of a doctor's office, including a laptop, cotton swabs, and tongue depressors'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a man riding a skateboard with neon green wheels'. Find frequency of the letter 'w'
Example Output:
2