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

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: 'some giraffes moving across a big grass field'. Find frequency of the letter 'e'

Ex Output:
3


Ex Input:
Sentence: 'a cheesy covered hot dog sits in a bun'. Find frequency of the letter 'a'

Ex Output:

2