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

Example Input: Sentence: 'a man riding a skateboard up the side of a ramp'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a bowl of granola with raspberries and honey on top with a spoon'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'a kitchen with many things on the counter top'. Find frequency of the letter 's'
Example Output:
1