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

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: 'one woman holding a video game controller and standing next to another woman'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a batter hits the ball with his bat'. Find frequency of the letter 'b'
Example Output:
3