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 man flying through the air on a bicycle'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'there is a man on the field playing with a frisbee'. Find frequency of the letter 'e'
Example Output:
6