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

Ex Input:
Sentence: 'a closeup view of a bathroom sink, that is very modern'. Find frequency of the letter 'e'

Ex Output:
4

Ex Input:
Sentence: 'a couple of people are sitting at a table'. Find frequency of the letter 'e'

Ex Output:
5

Ex Input:
Sentence: 'a green train engine sitting at the end of the train and next to the buidling'. Find frequency of the letter 'b'

Ex Output:
1