The sum of all of the probabilities have to be equal to 1. In that example with the dice, or let's say, since it's faster to draw the coin, the two probabilities have to be equal to 1. So if this is 1, 0, where x is equal to 1 if we're heads, or 0 if we're tails, each of these have to be 0.5. Or they don't have to be 0.5, but if 1 was 0.6, the other one would have to be 0.4. They have to add to 1. If one of these was, you can't have a 60% probability of getting a heads, and then a 60% probability of getting a tails as well, because then you would have essentially a 120% probability of either of the outcomes happening, which makes no sense at all. So it's important to realize that a probability distribution function, or probability distribution function in this case for a discrete random variable, they all have to add up to 1.