Geometric distribution
Keep tossing a coin, how likely to get the first heads at the $k$-th toss?
$$
p(k) =
\begin{cases}
(1-p)^{k-1}p, & \text{if $k = 1, 2, \cdots$,} \\\
0, & \text{otherwise.}
\end{cases}
\\[16pt]
\text{where $p$ is the probability the coin lands on heads.}
$$