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

Example Input: Sentence: 'a plastic container next to a toilet on a wooden box'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a bird on a tree is flapping its wings'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a group of people standing around a table covered in pizzas'. Find frequency of the letter 'e'
Example Output:
5