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

Example Input: Sentence: 'the miniature, elaborate exhibit depicts an ancient boating scene'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a man in a black shirt prepares to catch a frisbee'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a man putting some cheese on top of his pizza slice'. Find frequency of the letter 'g'
Example Output:
1