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

Example Input: Sentence: 'man is snow gear and skis skiing in the snow'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a woman in a tank top reaches out for a frisbee'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a bathroom with a toilet and a stand up shower'. Find frequency of the letter 's'
Example Output:
2