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

Ex Input:
Sentence: 'a man serving himself pizza in a wooden booth'. Find frequency of the letter 'n'

Ex Output:
4

Ex Input:
Sentence: 'a person that is looking out his window'. Find frequency of the letter 'n'

Ex Output:
3

Ex Input:
Sentence: 'a man sitting on top of a yellow fire hydrant'. Find frequency of the letter 'y'

Ex Output:
2