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

Example Input: Sentence: 'a jet that is flying in the sky with a banner behind it'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a person is water skiing and there are birds flying around above them'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a white and red bus is going down the street'. Find frequency of the letter 't'
Example Output:
4