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

Ex Input:
Sentence: 'a white police vehicle parked on a paved street'. Find frequency of the letter 'a'

Ex Output:
4

Ex Input:
Sentence: 'a family sitting at a table in a restaurant looking at their phones'. Find frequency of the letter 't'

Ex Output:
8

Ex Input:
Sentence: 'a young female wearing shorts on a skateboard'. Find frequency of the letter 's'

Ex Output:
3