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

Example Input: Sentence: 'a dilapidated bench in a park under a tree'. Find frequency of the letter 'p'
Example Output: 2

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

Example Input: Sentence: 'a piece of meat covered in ketchup next to a hamburger on a green plate'. Find frequency of the letter 'g'
Example Output:
2