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

Example Input: Sentence: 'a man riding a white surfboard on a large wave'. Find frequency of the letter 'a'
Example Output: 7

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 red fire truck sitting on the grass, near a pole'. Find frequency of the letter 'i'
Example Output:
3