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

Example Input: Sentence: 'a girl hitting a tennis ball on a court'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a man standing on a surf board in the ocean'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a stop sign by a mountain next to a road'. Find frequency of the letter 'a'
Example Output:
5