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

Example Input: Sentence: 'the receipts from a meal at a restaurant are for  a sandwich, chili, and apple'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a cyclist and bus splash past one another on the street'. Find frequency of the letter 'c'
Example Output:
2