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

Example Input: Sentence: 'a man makes some cock tales at a desk'. Find frequency of the letter 'k'
Example Output: 3

Example Input: Sentence: 'a person is performing a stunt on a snowboard'. Find frequency of the letter 'p'
Example Output: 2

Example Input: Sentence: 'a person playing lawn tennis in the court'. Find frequency of the letter 'y'
Example Output:
1