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 white bathroom sink sitting next to a toilet'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'two woman play wii video game with wireless controllers'. Find frequency of the letter 'w'
Example Output:
5