In combinatorics, a branch of mathematics, the inclusion–exclusion principle is a counting technique which generalizes the familiar method of obtaining the number of elements in the union of two finite sets; symbolically expressed as | A ∪ B | = | A | + | B | − | A ∩ B | {\displaystyle |A\cup B|=|A|+|B|-|A\cap B|} where A and B are two finite sets and |S | indicates the cardinality of a set S. Wikipedia