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

Example Input: Sentence: 'two baseball players high five each other during a game'. Find frequency of the letter 'y'
Example Output: 1

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

Example Input: Sentence: 'a couple of zebra walking across a field'. Find frequency of the letter 'o'
Example Output:
3