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

Example Input: Sentence: 'a herd of horses standing next to each other on a field'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a plate of food, with broccoli and nuts'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a big computer screen with a man on it'. Find frequency of the letter 'e'
Example Output:
3