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

Example Input: Sentence: 'eight donuts are cooked together in oil in a metal tray'. Find frequency of the letter 'h'
Example Output: 2

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: 'the man in the baseball cap is driving a boat'. Find frequency of the letter 'a'
Example Output:
6