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

Example Input: Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'this marketplace has a lot of fresh fruit and vegetables'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'street signs at the corner of dyer and west 31st street'. Find frequency of the letter 'h'
Example Output:
1