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

Ex Input:
Sentence: 'a photo of a couple of people at a convention'. Find frequency of the letter 'v'

Ex Output:
1

Ex Input:
Sentence: 'a person is performing a stunt on a snowboard'. Find frequency of the letter 'p'

Ex Output:
2

Ex Input:
Sentence: 'a fire hydrant with a hose sticking out of it's side'. Find frequency of the letter 't'

Ex Output:
5