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

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 child sitting a table with a pizza in front of him'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a wooden bench with stuffed animals sitting on top of it'. Find frequency of the letter 'h'
Example Output:
2