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

Example Input: Sentence: 'a large bathroom with marble floor, counter, and bath'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a truck is parked in front of a store'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'an all white bathroom with white towels sitting on the side wall'. Find frequency of the letter 'i'
Example Output:
6