Negative Binomial
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$r \geq 2$
$X - \text{# of trials until the rth success}$
$x \in \mathbb{Z}^+$
$x \in {r, ~r+1, ~r+2, \cdots}$
$$ P(X=x) = {{x-1} \choose {r-1}} p^{r-1} (1-p)^{x-r} p $$
Where ${{x-1} \choose {r-1}} p^{r-1} (1-p)^{x-r}$ is the $r-1$ successes out of x-1 trials, and $p$ is the $x$th trial of $r$th success.