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

Example Input: Sentence: 'young girl playing with refrigerator magnet while kneeling on floor'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a man opening up a package sitting on a couch'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a juicer attached to the top of a bike'. Find frequency of the letter 'h'
Example Output:
2