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

Example Input: Sentence: 'the street sign is next to a curved road'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'three young people goofing around; one of the guys has a football, the girl has a teddy bear'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a yellow car driving on the beach by the ocean'. Find frequency of the letter 'e'
Example Output:
5