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

Example Input: Sentence: 'a man holding a baseball bat an playing baseball'. Find frequency of the letter 'l'
Example Output: 6

Example Input: Sentence: 'the wooden kitchen has a small child's plastic table in it'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a group of people standing next to a bus and a flag pole'. Find frequency of the letter 'l'
Example Output:
3