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

Example Input: Sentence: 'a cat sprawls out and sleeps on its back on a lounge chair with an acorn theme'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a group of people standing on grass with a train in the background'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'police officers riding bikes in the city at night'. Find frequency of the letter 'i'
Example Output:
8