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

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: 'two people in a snowy place near a wooden fence'. Find frequency of the letter 'n'

Ex Output:
5

Ex Input:
Sentence: 'a bunch of different cupcakes that are going into the oven'. Find frequency of the letter 'e'

Ex Output:
6