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 red train traveling down train tracks near a forest'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a room with a fire place, chair, couch, two lamps, tables, television and ottomans'. Find frequency of the letter 'l'
Example Output:
4

