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

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 blue fire hydrant painted on the bottom of it'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a leather purse laying on it's side on a green metal bench'. Find frequency of the letter 'p'
Example Output:
1