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

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: 'girls posing of a picture one holding a tennis racket the other wearing a crown'. Find frequency of the letter 'g'
Example Output: 4

Example Input: Sentence: 'a zebra grazing on dry grass next to a stone wall'. Find frequency of the letter 'r'
Example Output:
4