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

Example Input: Sentence: 'this is an image of a woman playing tennis'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a notebook and pen are left on a park bench'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'there are many different foods in tis fridge'. Find frequency of the letter 'n'
Example Output:
3