To make it even worse, that's based on an average density 1.8 atoms/c3. The real values would probably vary wildly, and you won't be able to "break" in advance in order to go through a high density area.
And then there's the problem of hitting an interstellar grain of sand at 0.5c.
And then there's the problem of hitting an interstellar grain of sand at 0.5c.