Euler Phi Function of Non-Square Semiprime

Theorem
Let $n \in \Z_{>0}$ be a semiprime with distinct prime factors $p$ and $q$.

Then:
 * $\phi \left({n}\right) = \left({p - 1}\right) \left({q - 1}\right)$

where $\phi \left({n}\right)$ denotes the Euler $\phi$ function.