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

Example Input: Sentence: 'a white toilet sitting next to a bathroom sink'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a bunch of people are standing out in a field a red one has their arms out as a white ball is in the air'. Find frequency of the letter 'e'
Example Output: 9

Example Input: Sentence: 'a motorcycle is parked in front of a tower viewer overlooking water'. Find frequency of the letter 'g'
Example Output:
1