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

Example Input: Sentence: 'a dog is shown sitting on rocks by the beach'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'a construction sign is displayed on a street'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a zebra and a young deer grazing in the grass together'. Find frequency of the letter 't'
Example Output:
3