Solution (source code)

= Solution

This is the <geometric distribution> on $\{0,1,2,\ldots\}$ with success parameter $1/2$. Its <probability generating function> is a <geometric series>:
$$
\boxed{G_N(s)=\frac12\sum_{k=0}^\infty\left(\frac s2\right)^k=\frac1{2-s},\qquad |s|<2.}
$$