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

Ex Input:
Sentence: 'older man getting ready to swing his bat at a ball on a stand in front of a group of viewers'. Find frequency of the letter 'n'

Ex Output:
7

Ex Input:
Sentence: 'some food on plates next to each other on a table'. Find frequency of the letter 'e'

Ex Output:
6

Ex Input:
Sentence: 'a cat lays on top of an open and turned on laptop computer'. Find frequency of the letter 'p'

Ex Output:
5