Closing the Windows

A lab room has $N$ windows and $N$ people. The windows are numbered $1$ through $N$, and every window starts closed.

Person $i$ visits every window whose number is a multiple of $i$. At each of those windows, the person closes it if it is open and opens it if it is closed. Person $1$ acts first, then person $2$, and so on through person $N$. Count how many windows are open at the end.

For $N = 3$ the process runs like this.

1. Person $1$ opens windows $1, 2, 3$, the multiples of $1$. The state is (open, open, open).
2. Person $2$ closes window $2$, the multiple of $2$. The state is (open, closed, open).
3. Person $3$ closes window $3$, the multiple of $3$. The state is (open, closed, closed).

Exactly $1$ window is open at the end.

The first line contains $N$, the number of windows and also the number of people. ($1 \le N \le 2100000000$)

Print how many windows are open at the end.