Take 2i stones from one of the piles. Then, throw away i of them, and put the remaining i in the other pile. Here, the integer i (1 ≤ i)can be freely chosen as long as there is a sufficient number of stones in the pile.

是指从一堆中取出2i个石子，将其中i个扔掉，剩下的i个放到另一堆里（前提是该堆里有至少2i个石子）