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

Example Input: Sentence: 'a person standing operating a cell phone in a room'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a airplane view of a big factory some people and cars'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a cake in the shape of a toilet with andy 50 today written on it'. Find frequency of the letter 'h'
Example Output:
3