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

Example Input: Sentence: 'a man paddle boarding by a boat dock in a city'. Find frequency of the letter 'o'
Example Output: 3

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

Example Input: Sentence: 'a ferry boat passing next to an island'. Find frequency of the letter 's'
Example Output:
3