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

Example Input: Sentence: 'a couple of girls are hanging out at a restaurant'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a living room and a kitchen are empty'. Find frequency of the letter 'a'
Example Output:
4