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

Example Input: Sentence: 'a little boy that has a toothbrush in his mouth'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a airplane view of a big factory some people and cars'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a group of people standing around a table with plates of food'. Find frequency of the letter 'r'
Example Output:
2