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

Example Input: Sentence: 'a double parking meter is decorated with a picture of a girl with wings on her feet'. Find frequency of the letter 'r'
Example Output: 6

Example Input: Sentence: 'a bowl of granola, dried fruit, and bananas'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'an egg on top of half a pizza'. Find frequency of the letter 'f'
Example Output:
2