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

Example Input: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'a close up of a train on a track in the dark'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a white plate topped with different flavored donuts'. Find frequency of the letter 'e'
Example Output:
6