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

Example Input: Sentence: 'a dog is shown sitting on rocks by the beach'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'a girl playing with a frisbee while people watch'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a red and a black laptop on a round black table near two people sitting down'. Find frequency of the letter 'n'
Example Output:
6