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: 'apples, pears, and oranges are piled up for sale'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a little boy about to hit a tennis ball '. Find frequency of the letter 'b'
Example Output:
3