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

Example Input: Sentence: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a picture of a toilet and toilet paper in a bathroom'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a few people are getting of a plane'. Find frequency of the letter 'f'
Example Output:
2