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

Example Input: Sentence: 'a plastic plastic plate topped with three different type of food'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a double parking meter is decorated with a picture of a girl with wings on her feet'. Find frequency of the letter 'r'
Example Output: 6

Example Input: Sentence: 'a woman wearing striped shirt and jeans sitting on a bench'. Find frequency of the letter 'n'
Example Output:
7