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

Example Input: Sentence: 'a dog sitting in the back of a truck flat bed'. Find frequency of the letter 'h'
Example Output: 1

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 container of noodles is next to another container with food'. Find frequency of the letter 'n'
Example Output:
7