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

Example Input: Sentence: 'a bunch of people are standing out in a field a red one has their arms out as a white ball is in the air'. Find frequency of the letter 'e'
Example Output: 9

Example Input: Sentence: 'a desk covered in notepads, a laptop and other devices'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'a man in red jacket in ski gear at the top of a slope'. Find frequency of the letter 'a'
Example Output:
6