So your chances of getting either not all tails or all tails, and these are mutual exclusives, so we can add them, so the probability of not all tails or the probability of all tails is going to be equal to 1. These are mutual exclusives. You're either going to have not all tails, which means a head shows up, or you're going to have all tails, but you can't have both of these things happening. And since they're mutual exclusives, and you're saying the probability of this or this happening, you can add their probabilities. And this is essentially all of the possible events. So this is essentially, if you combine these, this is the probability of any of the events happening, and that's going to be a 1 or 100% chance. So another way to think about it is the probability of not all tails is going to be 1 minus the probability of all tails.