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 group of people walking down a rain soaked street with umbrellas'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'a passenger train on the tracks in a rural area'. Find frequency of the letter 'a'
Example Output:
8