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

Example Input: Sentence: 'people playing wii bowling in a modern library'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a notebook and pen are left on a park bench'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a black and white young cat stands in the driver's seat window of a commercial van'. Find frequency of the letter 'm'
Example Output:
2