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: 'people seated on a table looking very happy'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a table with a keyboard cup and many items and some chairs'. Find frequency of the letter 'l'
Example Output:
1