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

Example Input: Sentence: 'a tv in a glass mirror in the bathroom'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a young man with glasses on is taking a big bite'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a yellow train coming down the train track'. Find frequency of the letter 'w'
Example Output:
2