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

Example Input: Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a airplane view of a big factory some people and cars'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'an indian elephant yaws as its trunk is sprayed'. Find frequency of the letter 'i'
Example Output:
4