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

Example Input: Sentence: 'a boy catching a frisbee while standing ins bunch of leaves'. Find frequency of the letter 'a'
Example Output: 5

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: 'a pastry store with many cupcakes on display'. Find frequency of the letter 'p'
Example Output:
3