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

Example Input: Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a person with some scissors over a bowl'. Find frequency of the letter 's'
Example Output: 6

Example Input: Sentence: 'a large bathroom with columns next to the tub'. Find frequency of the letter 'a'
Example Output:
3