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 player swings his bat at a baseball game'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'an octopus kite is being flown next to a scuba diver kite above a city park'. Find frequency of the letter 'i'
Example Output:
6