And if we take a simple example where we have this survival rate from one year to the next-- and I've set this equal to 0, so that nobody makes it to age 3-- starting at birth everybody--we're keeping track of things that get born-- so everybody has a probability of 1 of being born-- because those are the only ones we're counting-- and 50% of making it to age 1, and 25% of making it to age 2. 