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

Example Input: Sentence: 'a man in his car eating a donut'. Find frequency of the letter 'm'
Example Output: 1

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

Example Input: Sentence: 'a little dog standing on its back legs on a bed'. Find frequency of the letter 'b'
Example Output:
2