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

Example Input: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a man riding his skateboard down low as he turns'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a man stands next to a double-decker bus with bikes parked in front of it'. Find frequency of the letter 'k'
Example Output:
3