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

Example Input: Sentence: 'a large cooked pizza on a dining table'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a large bathroom with marble floor, counter, and bath'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'herd of sheep wandering around rocky area in countryside'. Find frequency of the letter 'n'
Example Output:
5