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

Example Input: Sentence: 'a truck with a camper sits parked in some vegetation'. Find frequency of the letter 's'
Example Output: 3

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 guy in a black wet suit is riding the waves on his surfboard'. Find frequency of the letter 'e'
Example Output:
3