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 man in his car eating a donut'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'buses driving down a road with buildings on the sides'. Find frequency of the letter 'e'
Example Output:
3