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

Example Input: Sentence: 'some cubbies with towels in a blue bathroom'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'street sign at the corner of spencer and 6th street'. Find frequency of the letter 'p'
Example Output:
1