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

Example Input: Sentence: 'a small dog sits in the rear window of a car'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'silhouette of person playing frisbee with moutain and sunset'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'pizza on a plate with a fork and knife'. Find frequency of the letter 'l'
Example Output:
1