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

Example Input: Sentence: 'a young woman dressed in green and black operating a remote control'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'large grassy pasture filled with trees and a single horse'. Find frequency of the letter 'l'
Example Output: 4

Example Input: Sentence: 'a young person riding a skateboard about to preform a trick'. Find frequency of the letter 'a'
Example Output:
6