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

Example Input: Sentence: 'a man in costume talking on a cell phone'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a man serving himself pizza in a wooden booth'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a room full of assorted glass jars around a blue chair'. Find frequency of the letter 's'
Example Output:
5