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 group of people are playing soccer on a beach'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a woman stands in the surf with a surfboard'. Find frequency of the letter 'd'
Example Output:
2