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

Example Input: Sentence: 'a piece of bread stuffed with meat and vegetables'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'the snowboarder wearing a dark helmet sprays snow into the air'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'a ginger cat lounges comfortably on a bed'. Find frequency of the letter 'n'
Example Output:
3