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

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

Example Input: Sentence: 'a professional baseball player stands at home plate with his bat'. Find frequency of the letter 'n'
Example Output:
2