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

Example Input: Sentence: 'a bird is perched on a container in a person's hand'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'a piece of meat covered in ketchup next to a hamburger on a green plate'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a man is water skiing on a river'. Find frequency of the letter 'r'
Example Output:
3