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

Ex Input:
Sentence: 'a truck with a camper sits parked in some vegetation'. Find frequency of the letter 's'

Ex Output:
3

Ex Input:
Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'

Ex Output:
2

Ex Input:
Sentence: 'teddy bear above a female baby i'm crib sleeping'. Find frequency of the letter 'y'

Ex Output:
2