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

Example Input: Sentence: 'many different species of sheep in display at a museum'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'a photo of a small boat on a shore'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a woman in a dress watching two people joust'. Find frequency of the letter 's'
Example Output:
3