Cardinality of finite sets is given by

\begin{align*} |A \cup B| & = |A| + |B| - |A \cap B| \\ |A \cup B \cup C| & = |A| + |B| + |C| - |A \cap B| - |A \cap C| - |B \cap C| + |A \cap B \cap C|. \end{align*}