5. Red Martians always tell the truth, while blue Martians lie and then turn red. In a company of 2018 Martians, each in turn answered the question of how many red ones there were among them. The answers were the numbers $1,2,3, \ldots, 2018$ (in that exact order). How many red ones could there have been initially?
Solution. Answer: 0 or 1

Let's prove that there cannot be two or more red individuals. Suppose they do exist, and let's look at the first two red individuals. The difference in their answers is the number of people who spoke between them, +1. This means that the number of red individuals should have changed by this number. However, only those who spoke between them could have turned red, and there is one fewer of them - a contradiction.

If all the blue individuals are blue, or all but the first one, then everything fits: the blue individuals assert that there is one more red individual than the actual number at that moment (in the variant with the first one being red, he speaks the truth that there is 1 red).