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

Example Input: Sentence: 'two pepperoni pizzas and a cup on a counter'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'a jet that is flying in the sky with a banner behind it'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a man picks up a donut off of a platter'. Find frequency of the letter 's'
Example Output:
1