Let $X_1, \ldots, X_N$ each be a D17: Finite set.

Then \begin{equation} \left| \bigcup_{n = 1}^N X_n \right| = \sum_{J \subseteq \{ 1, \ldots, N \} : J \neq \emptyset} (-1)^{|J| + 1} \left| \bigcap_{j \in J} X_j \right| \end{equation}