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

Example Input: Sentence: 'a man holding a frisbee high into the air'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a couple of men standing inside of a liquor store'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'a plate of food including fries and mac and cheese'. Find frequency of the letter 'a'
Example Output:
5