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

Example Input: Sentence: 'a small white plane sitting on top of the ocean'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a room with a television, a vase, two suitcases and a trunk'. Find frequency of the letter 's'
Example Output:
5