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: 'a person is holding a chocolate doughnut with rainbow sprinkles'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a baseball player standing next to home plate'. Find frequency of the letter 'o'
Example Output:
2