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 passenger train that is going over a bridge'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'there is no image here to provide a caption for'. Find frequency of the letter 'o'
Example Output:
5