Length of Subgroup Plus Length of Quotient Group

Theorem
Let $G$ be a finite group.

Let $H$ be a normal subgroup of $G$.

Then:
 * $\map l G = \map l H + \map l {G / H}$

where:
 * $\map l G$ denotes the length of $G$
 * $G / H$ denotes the quotient group of $G$ by $H$.