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

Ex Input:
Sentence: 'a young boy bouncing a tennis ball on a tennis racquet'. Find frequency of the letter 'a'

Ex Output:
5


Ex Input:
Sentence: 'a man holding a brown paper bag kite'. Find frequency of the letter 'b'

Ex Output:
2


Ex Input:
Sentence: 'a computer desk with a computer and printer on it'. Find frequency of the letter 'r'

Ex Output:

4