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

Example Input: Sentence: 'a photo of a small boat on a shore'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'the sun goes down over a bike tied to a bench at the beach'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a modern building has a long table holding many fruits and vegetables'. Find frequency of the letter 'n'
Example Output:
6