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

Ex Input:
Sentence: 'a person that is looking out his window'. Find frequency of the letter 'n'

Ex Output:
3


Ex Input:
Sentence: 'a person riding a skate board on a city street'. Find frequency of the letter 'a'

Ex Output:
5


Ex Input:
Sentence: 'a man throws his bat on a field'. Find frequency of the letter 'f'

Ex Output:

1