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

Example Input: Sentence: 'the small black and white dog holds onto a frisbee'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a large formal living room with yellow couches and table'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a passenger bus parked in a parking lot'. Find frequency of the letter 'i'
Example Output:
2