Ms.Han was addicted to games about light .
There are N lights putting in a line S[1...N] and we can change their status (open and closed ) by pressing their button. But they have a characteristic, when you press light s[i] (0<=i<=N), those lights who's numbers are i's multiple will also change their status.For example:There are 10 lights s[1...10] whose initial status is closed. When she press the light s (it will be open), then the light s,s,s,s and s will also be open. When she press s again, light s and s will be closed.Now Ms.Han has N lights all closed ,she will press them from 1 to N ,then how many lights will be open in the end?
10 15 20
3 3 4