$\displaystyle \frac{(-1)^n}{(n!)^2z^n}\\$
z is a complex number
$\displaystyle \frac{1}{R}=\lim_{n->\infty}\sqrt[n]{|c_n|}\\$
$\displaystyle \frac{1}{R}=\frac{1}{\sqrt[n]{(n!)^2z^n}}\\$

what to do next?