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

Example Input: Sentence: 'two laptop computers sitting on top of a white table'. Find frequency of the letter 'f'
Example Output: 1

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 table full of empty wine glasses and silverware'. Find frequency of the letter 'a'
Example Output:
5