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

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: 'one woman holding a video game controller and standing next to another woman'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a city at night with a green traffic signal'. Find frequency of the letter 'f'
Example Output:
2