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

Example Input: Sentence: 'a ready to eat pizza on a rack'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a close up of a train on a track in the dark'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a man hits a tennis ball with a racket on the court'. Find frequency of the letter 'c'
Example Output:
2