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

Example Input: Sentence: 'a couple is cutting in to a cake at a wedding'. Find frequency of the letter 'd'
Example Output: 2

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 tray containing a variety of food items representing a full course meal'. Find frequency of the letter 'f'
Example Output:
3