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: 'person wearing all white leaned up against a wall with a yellow sign'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a cat drinking from a coffee cup next to loaves of bread'. Find frequency of the letter 'g'
Example Output:
1