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 dog on a couch near many pillows'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'a lonely old grey abandoned boarded up motel'. Find frequency of the letter 'a'
Example Output:
4