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

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

Ex Output:
1

Ex Input:
Sentence: 'two young ladies that are dancing in the room together'. Find frequency of the letter 'a'

Ex Output:
4

Ex Input:
Sentence: 'two shirtless men are playing with frisbees outside'. Find frequency of the letter 's'

Ex Output:
6