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

Example Input: Sentence: 'a guy surfs a wave with his surfboard'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a few men walking through a market place'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a small wrinkley dog wearing a bow tie'. Find frequency of the letter 'e'
Example Output:
3