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

Example Input: Sentence: 'a woman and a child walking down a street with a kite'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'the street sign is next to a curved road'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'the kitchen is clean and ready to be cooked in'. Find frequency of the letter 'n'
Example Output:
4