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

Example Input: Sentence: 'two elephants facing each other touching trunks in an enclosure'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a group of people having dinner and drinking wine'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a room with a large window table and chairs a bookcase'. Find frequency of the letter 'm'
Example Output:
1