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

Example Input: Sentence: 'a large formal living room with yellow couches and table'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a toilet sits in a cluttered bathroom next to a sink'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a pair of red flowers in a green vase'. Find frequency of the letter 'd'
Example Output:
1