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

Example Input: Sentence: 'a microwave oven with a yellow note on the door'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'a bunch of green bananas is hanging on a tree'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a young man posing for a picture alone in a field'. Find frequency of the letter 'l'
Example Output:
2