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

Example Input: Sentence: 'a dilapidated bench in a park under a tree'. Find frequency of the letter 'p'
Example Output: 2

Example Input: Sentence: 'a boy that is sitting at a table with pizza'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'two biscuits with frosting on top displayed on a white plate'. Find frequency of the letter 'u'
Example Output:
1