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

Example Input: Sentence: 'a blue bowl holding mashed bananas and oatmeal with a bottle of sugar and cinnamon sitting beside it'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'a dog is shown sitting on rocks by the beach'. Find frequency of the letter 'a'
Example Output: 2

Example Input: Sentence: 'a snowboarder is seen in mid air doing a trick'. Find frequency of the letter 'n'
Example Output:
4