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

Example Input: Sentence: 'different sets of luggage in a baggage claim area'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a man with headphones on is hunched over a sink while being recorded'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'a red and white double layer bus driving down a street'. Find frequency of the letter 'b'
Example Output:
2