C357 Perfectly Puzzled

When he just climbed up to the second floor, he was immediately startled by a sound, and couldn't help but freeze on the spot, holding on to the banister of the staircase. After a while, as if recognizing the sound, Chen Nan revealed a smile and said, "Isn't it a little too early now?" As he continued to walk towards the fourth floor, just as Chen Nan went up to the second floor