An errand boy was collecting boxes of cakes for the Summer Fair. He collected boxes from various people in his local village and each box was labelled in Roman Numerals with the number of cakes in the box. By the time the errand boy had collected the last box, he was quite hungry, and really needed to eat at least one cake. Luckily the last box was marked with an underlined XI, meaning there were 11 cakes in it. He had the brain wave of turning the box around and underlining the number again to give the impression there were IX, that is 9 cakes. However, after eating the 2 cakes, he was still hungry. How can he change the number shown on the box again and eat more cakes?