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

Example Input: Sentence: 'a sailboat is floating near the rocky shore of a beach'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a group of people having dinner and drinking wine'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a little boy holding a blue kite while standing in a lush green field'. Find frequency of the letter 'e'
Example Output:
7