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

Example Input: Sentence: 'a passenger bus that is driving down the street'. Find frequency of the letter 'r'
Example Output: 3

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

Example Input: Sentence: 'a boy doing a trick on a skateboard over a rail'. Find frequency of the letter 'y'
Example Output:
1