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

Example Input: Sentence: 'a man with headphones on is hunched over a sink while being recorded'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'two computers and a laptop set up on a desk'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a white toilet sitting next to a toilet paper roller'. Find frequency of the letter 'o'
Example Output:
4