<image>Andy fills a $3 \times 3$ table with all the digits from 1 to 9 so that each cell only contains one digit. He has already put the digits 1, 2, 3 and 4 in the table as shown in the diagram. Two numbers are 'neighbouring' when the cells they are in share one side. After he had finished filling in the table he noticed: The sum of the numbers neighbouring 9 equals 15. How big is the sum of the numbers neighbouring 8?