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

Example Input: Sentence: 'a man walking a large elephant down a dirt road'. Find frequency of the letter 'g'
Example Output: 2

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: 'two children who are standing next to a white fire hydrant'. Find frequency of the letter 'i'
Example Output:
4