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

Example Input: Sentence: 'a parking meter painted green with a sign that describes it as not a parking meter'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'an image of a cautionary construction sign'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a pot on top of a stove next to a large metal ladle'. Find frequency of the letter 'n'
Example Output:
2