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

Example Input: Sentence: 'a glass bowl of raw broccoli, a ceramic bowl of raw green chili peppers, and steel bowl of raw daikon'. Find frequency of the letter 'a'
Example Output: 9

Example Input: Sentence: 'a person working at an airport, outside of an airplane'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
Example Output:
3