I timed it and this runs about 4X faster than the for loop solution on input of 100,000,000. But being relatively new to Python, I'd have to research what this is doing to understand it; comments are definitely needed.

I agree w/ @sine that a variable named 'sum' isn't wise. That withstanding, my solution was the same. This solution consistently runs about a half second faster than the higher voted one-liner with input of 100,000,000, and I'd argue it is easier to read.