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

Example Input: Sentence: 'a street at night with trees and streetlights on'. Find frequency of the letter 't'
Example Output: 9

Example Input: Sentence: 'a bunch of broken down trains sit in a yard'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'an grey elephant roaming around the rocks outside'. Find frequency of the letter 'd'
Example Output:
2