Why do the probability of an event and the probability of its complement add up to 1

the probability of an event and the probability of its complement add up to 1 because the total events are the even it self and its compliment, so its probability equal to 1 because these are all the possible events that will occur. for example a coin toss, an event head will happen 0.5, and its complement is tails which will also happens 0.5