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

Example Input: Sentence: 'a pizza pie sitting on a wooden table with tongs'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'slices of orange are arranged on a plate'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a trolley bus is coming down the street near trees'. Find frequency of the letter 'r'
Example Output:
4