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: 'a group of people seated in chairs together and facing the same direction, with one man holding a soccer ball in his hands'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'many airport employees work around a plane on a runway'. Find frequency of the letter 'o'
Example Output:
5

