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

Example Input: Sentence: 'people waiting on a train platform in a wooded area'. Find frequency of the letter 'p'
Example Output: 3

Example Input: Sentence: 'a boy that is sitting at a table with pizza'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'small child about to take a bite of her breakfast food'. Find frequency of the letter 'k'
Example Output:
2