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

Example Input: Sentence: 'a woman in a messy kitchen with a child'. Find frequency of the letter 'a'
Example Output: 4

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 white bathroom sink sitting next to a toilet'. Find frequency of the letter 'i'
Example Output:
5