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

Example Input: Sentence: 'a woman walking on a shore next to a river'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a couple of buses parked on the side of the street'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a white toilet sitting underneath a window in a bathroom'. Find frequency of the letter 'n'
Example Output:
5