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

Example Input: Sentence: 'a ready to eat pizza on a rack'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'three people stand on the back of three elephants'. Find frequency of the letter 'r'
Example Output:
2