Counting Outcomes That Map to the Same Value

Two fair six-sided dice — one red, one blue — are rolled together. All $36$ ordered outcomes $(a, b)$ are equally likely, where $a$ is the red die and $b$ is the blue die.

Define the random variable $X = |a - b|$

so $X$ is the size of the gap between the two faces.

What is $P(X = 1)$?