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

Example Input: Sentence: 'a batter, catcher and umpire waiting for the pitch and a major league baseball game'. Find frequency of the letter 'c'
Example Output: 3

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

Example Input: Sentence: 'a white toilet is on the street by a pole'. Find frequency of the letter 'e'
Example Output:
6