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

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 large formal living room with yellow couches and table'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a young girl walking on a road carrying an umbrella'. Find frequency of the letter 'l'
Example Output:
4