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

Example Input: Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'there is a zebra that is walking enar a car'. Find frequency of the letter 'r'
Example Output: 4

Example Input: Sentence: 'a bottle of wine and a wine glass on a table'. Find frequency of the letter 's'
Example Output:
2