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

Example Input: Sentence: 'a woman holds the reins of a saddled horse'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a black animal in dirt area below trees and rocks'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a white plat topped with fence toast and a cup of syrup'. Find frequency of the letter 'a'
Example Output:
5