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: 'bathroom scene with a black cat going behind the door'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a chef is leaning on his counter with green jars on it'. Find frequency of the letter 'n'
Example Output:
6