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

Example Input: Sentence: 'decorated baseball cake in a clear plastic box'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a crowd of fans watch soccer players from stadium seating'. Find frequency of the letter 'f'
Example Output: 3

Example Input: Sentence: 'a person standing with a luggage bag with a handle'. Find frequency of the letter 'p'
Example Output:
1