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

Example Input: Sentence: 'a photo of a toilet which is not clean'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a man riding on the back of a white horse next to other horses'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'the tennis player is reaching up towards the tennis ball'. Find frequency of the letter 'a'
Example Output:
4