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

Example Input: Sentence: 'a person trying to flip a skateboard in a garage'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a man holding a brown paper bag kite'. Find frequency of the letter 'b'
Example Output: 2

Example Input: Sentence: 'a black bull lays outside in the snow'. Find frequency of the letter 'l'
Example Output:
4