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

Ex Input:
Sentence: 'a black animal in dirt area below trees and rocks'. Find frequency of the letter 'n'

Ex Output:
3

Ex Input:
Sentence: 'a piece of meat covered in ketchup next to a hamburger on a green plate'. Find frequency of the letter 'g'

Ex Output:
2

Ex Input:
Sentence: 'three double decker busses cruise down the street'. Find frequency of the letter 'w'

Ex Output:
1