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

Example Input: Sentence: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
Example Output: 3

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 cat laying onto of an unzipped suitcase'. Find frequency of the letter 'o'
Example Output:
3