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

Example Input: Sentence: 'a teenager witting on a futon couch holding gaming controls in both hands'. Find frequency of the letter 'c'
Example Output: 3

Example Input: Sentence: 'a little boy that has a toothbrush in his mouth'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a colorful passenger train pulled by a green engine'. Find frequency of the letter 'r'
Example Output:
4