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

Input: Consider Input: Sentence: 'a double decker bus with a lot of people on board'. Find frequency of the letter 'l'

Output: 3

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

Output: 3

Input: Consider Input: Sentence: 'a picture of of martin luther king jr. standing in front of the lincoln memorial'. Find frequency of the letter 'f'
Output: 4