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

Example Input: Sentence: 'an elephant has taken a man's hat in its trunk'. Find frequency of the letter 'p'
Example Output: 1

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 man holding a tennis racquet over his right shoulder'. Find frequency of the letter 'r'
Example Output:
4