11584: 计算机C13001第一次小组赛C题

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[2] (it will be open), then the light s[2],s[4],s[6],s[8] and s[10] will also be open. When she press s[4] again, light s[4] and s[8] 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