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

Example Input: Sentence: 'a commuter bus in england by a clock tower building'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'there is a zebra looking at a camera in dry grass'. Find frequency of the letter 'a'
Example Output:
7