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

Example Input: Sentence: 'a small doughnut held in a persons hand'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a desk covered in notepads, a laptop and other devices'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'sleeping cat snuggled up to a pair of sneakers'. Find frequency of the letter 'e'
Example Output:
5