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

Ex Input:
Sentence: 'a photo of a small boat on a shore'. Find frequency of the letter 'f'

Ex Output:
1

Ex Input:
Sentence: 'the receipts from a meal at a restaurant are for a sandwich, chili, and apple'. Find frequency of the letter 'u'

Ex Output:
1

Ex Input:
Sentence: 'a group of dogs standing outside of a screen door'. Find frequency of the letter 'o'

Ex Output:
7