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 working at an airport, outside of an airplane'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a knife a cutting board some bananas and a toaster'. Find frequency of the letter 'i'
Example Output:
2