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

Example Input: Sentence: 'a large bathroom with marble floor, counter, and bath'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a kitchen with hanging lights, an island with sink and bar stools, a refrigerator, small pantry door, wooden cabinets and shiny wood floor'. Find frequency of the letter 'n'
Example Output: 12

Example Input: Sentence: 'a surfer wipes out on a huge wave'. Find frequency of the letter 'a'
Example Output:
3

