N people sit at a round table, starting from #1, every other one leaves, who's the last one?

N people sit at a round table, starting from #1, every other one leaves,
who's the last one?

For example, there are 10 people sitting there.
So the 1st round, such people leave: $$\#1, \#3, \#5, \#7, \#9$$ and
remains $$\#2, \#4, \#6, \#8, \#10$$
Then the 2nd round, such people leave: $$\#2, \#6, \#10$$ and remains
$$\#4, \#8$$
Then the 3rd round, such people leave: $$\#8$$ and remains $$\#4$$
so the last one remained is #4.
If we note f(10)=4, how to get a general formula for f(N)?