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

Example Input: Sentence: 'a group of giraffes walk together by a hill'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a person trying to flip a skateboard in a garage'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a close up of a person grabbing a pastry in a container'. Find frequency of the letter 'i'
Example Output:
3