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

Example Input: Sentence: 'a cat sprawls out and sleeps on its back on a lounge chair with an acorn theme'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a young girl is brushing her teeth in this dated photo'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a little girl holds something in her hand next to a christmas stocking'. Find frequency of the letter 't'
Example Output:
7