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

Example Input: Sentence: 'a half eaten doughnut in a persons hand'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'two people are riding on canoes down a river'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a big black dogs lays on a bench next to a trash bag'. Find frequency of the letter 'a'
Example Output:
7