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

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

Example Input: Sentence: 'a man with a skateboard that is in the street'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a man sitting in a living room holding a brown cat'. Find frequency of the letter 'r'
Example Output:
2