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

Example Input: Sentence: 'a couple of buildings that has a bus out front'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a man walking a large elephant down a dirt road'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a white dog with black spots sits in the grass'. Find frequency of the letter 's'
Example Output:
6