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

Example Input: Sentence: 'man in a sleeping bag on the floor using a laptop computer'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a large motorcycle that has a man dressed in old clothing next to it'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a woman stands behind her luggage with her turtle neck pulled halfway over her face'. Find frequency of the letter 'f'
Example Output:
2