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

Example Input: Sentence: 'there is a person standing on a blue towel'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a black cat laying down on a bed'. Find frequency of the letter 'y'
Example Output: 1

Example Input: Sentence: 'a bathroom with tiled floor and a toilet'. Find frequency of the letter 'l'
Example Output:
3