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: 'men standing in the grass with a frisbee'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'a table with several unique dishes and desserts on it'. Find frequency of the letter 'e'
Example Output:
7