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

Example Input: Sentence: 'a little dog on a table while a dog groomer cuts its fur'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'one man grabbing another at a table while laughing'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a man at a bar tasting wine that is lined out on the bar counter top'. Find frequency of the letter 't'
Example Output:
9