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

Example Input: Sentence: 'a person is performing a stunt on a snowboard'. Find frequency of the letter 'p'
Example Output: 2

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 flying through the air while riding a skateboard'. Find frequency of the letter 'd'
Example Output:
2