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

Example Input: Sentence: 'a double decker bus with a lot of people on board'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a fluffy white car looking in a mirror'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a  young person flying a kite at the beach'. Find frequency of the letter 'g'
Example Output:
2