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

Example Input: Sentence: 'the meal is served and ready to be eaten'. Find frequency of the letter 'e'
Example Output: 8

Example Input: Sentence: 'a yellow fire hydrant in the forefront of a roadway'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a modern commuter bus at a bus stop on a city street'. Find frequency of the letter 'e'
Example Output:
4