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

Ex Input:
Sentence: 'two zebras standing around while a third zebra drinks from a pond'. Find frequency of the letter 's'

Ex Output:
3


Ex Input:
Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'

Ex Output:
2


Ex Input:
Sentence: 'an image of a vase next to a pair of glasses'. Find frequency of the letter 'a'

Ex Output:

7