Four towns are connected by five roads. Imagine a rhombus where the left, up, right, and down corners are $A, B, D$, and $C$ respectively; and connect the corners B and C. Each road is blocked by snow independently with probability $p$. What is the probability that you can drive between the towns $A$ and $D$?
I will explain how I understand the first part of the solution.
More importantly, I am asking for someone to explain the solution which is below.
This question is taken from "Probability and Random Variables, a Beginner's guide" by David Stirzaker, pages $63-64$, which shows the following solution:
Let $R$ be the event that the road BC is open and Q the event that you can drive from $A$ to $D$.
$R'$ means the complement of event $R$.
$$P(Q)=P(Q|R) \cdot (1−p)+P(Q|R′)\cdot p$$
$$P(Q)=(1−p^2)^2\cdot (1−p)+(1−(1−(1−p)^2)^2)\cdot p$$
I now will explain my understanding of the first part of the above solution:
(1) $p^2$ is the probability of having two consecutive roads (such as $AB$ and $BD$; or $AC$ and $CD$; or $AB$ and $BC$; or $AC$ and $CB$) blocked. Note that in the case of $AB$ and $BC$, or $AC$ and $CB$, the town $D$ was not reached.
(2) $1-p^2$ is the probability that two consecutive roads are not blocked, and $(1-p^2)^2$ is the probability that two pairs of consecutive roads are not blocked, such as $AB$ and $BD$; or $AC$ and $CD$.
(3) $(1-p^2)^2 \cdot (1-p)$ is the probability that two consecutive roads are not blocked plus road $BC$ not being blocked. So that means the routes $AB$ and $BC$ and $CD$; or $AC$ and $CB$ and $BD$. The problem with this solution is that, apparently, it ignores the routes $AB$ and $BD$; or $AC$ and $CD$.
That was my attempt at understanding the first term of the solution. I invite an explanation of the book's solution (both, the first and second terms of the solution) and, ideally, show step by step how such solution was found, hopefully showing equations with unions and intersections of events.