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

Ex Input:
Sentence: 'a small white dog looking out a car window'. Find frequency of the letter 'l'

Ex Output:
3


Ex Input:
Sentence: 'small black bird with red on its head standing on a tree branch'. Find frequency of the letter 'b'

Ex Output:
3


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

Ex Output:

5