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

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 young woman dressed in green and black operating a remote control'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a toilet and shower is inside of a bathroom'. Find frequency of the letter 'o'
Example Output:
5