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

Example Input: Sentence: 'a notebook and pen are left on a park bench'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a train station with a train pulling into the platform'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a giraffe standing next to a wall in an enclosure'. Find frequency of the letter 'e'
Example Output:
4