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

Example Input: Sentence: 'two giraffes standing next to some trees'. Find frequency of the letter 'f'
Example Output: 2

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: 'four people doing various skiing tricks on a ski slope'. Find frequency of the letter 'e'
Example Output:
3