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

Example Input: Sentence: 'three birds are sitting on a tree in a forest'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'a cooked pizza on a silver platter with another in the background'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a person on a skateboard in the street'. Find frequency of the letter 'b'
Example Output:
1