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

Example Input: Sentence: 'an audience watches as a bird spreads its wings'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a black cat laying down on a bed'. Find frequency of the letter 'y'
Example Output: 1

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