Principle of Inclusion-Exclusion (PIE)

Given two finite sets, we can find the cardinality of their union by adding the two individual cardinalities and subtracting the cardinality of the intersection. But how can we find the cardinality of the union of $n$ finite sets? This is the general principle of inclusion-exclusion, otherwise known as PIE. We state this general formula and mention its symmetric form in this video.