My AC program must have work time propoprtional to V^3.
V is at most 222 in this task, but server writes that my program worked only 0.015 sec. How is this possible? I suppose, this indication has no relation at all to amount of time my program actually took to process the most difficult test.. Am I right?

Test data is not rather good, but V^3 = 10 mln. is not many operations actually. So it's not surprising, that your program works fast.