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

Example Input: Sentence: 'a little boy sitting on a bed in a bedroom'. Find frequency of the letter 'e'
Example Output: 3

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 food processor has a small amount of flour in it'. Find frequency of the letter 'n'
Example Output:
2