Solution
= Solution
Choose
$$
\boxed{\alpha_1=\varepsilon_1-\varepsilon_2,\qquad
\alpha_2=\varepsilon_2}.
$$
Here $\alpha_1$ is long and $\alpha_2$ is short. The <Dynkin diagram> consists of two vertices joined by a double edge, with its arrow pointing from $\alpha_1$ toward the shorter root $\alpha_2$:
$$
\alpha_1\Longrightarrow\alpha_2.
$$