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

Example Input: Sentence: 'the large modern train is on the train track'. Find frequency of the letter 'm'
Example Output: 1

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 desk with a laptop, computer, headphones and a book on it'. Find frequency of the letter 'e'
Example Output:
4