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

Example Input: Sentence: 'a man in a black shirt prepares to catch a frisbee'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'three men are in a kitchen cooking and cutting through a dish'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'some sort of sports athlete holding her fist up, holding an object'. Find frequency of the letter 'e'
Example Output:
5