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

Example Input: Sentence: 'a doorway looking into a small bathroom with white toilet'. Find frequency of the letter 'r'
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 large building with a clock affixed to the top of it'. Find frequency of the letter 'd'
Example Output:
2