Thought I broke through Level 4, but...

I reached Level 5 in the blink of an eye. So there are high-difficulty levels and low-difficulty levels, huh? Considering the previous high-difficulty level, are they a set of high-difficulty -> low-difficulty? Does that mean the odd-numbered levels are the high-difficulty ones? Oh, I really leveled up quickly. It probably didn't even take 2 minutes. I leveled up to 5 with a bit of surprise. It took me 3 days to break through Level 4, though.