Solution
= Solution
The <root string> through $\alpha_1$ in the $\alpha_2$ direction contains $\alpha_1$ and $\alpha_1+\alpha_2$, so $\alpha_1$ can be raised once by $\alpha_2$. The string through $\alpha_2$ in the $\alpha_1$ direction contains
$$
\alpha_2,\quad\alpha_2+\alpha_1,\quad
\alpha_2+2\alpha_1,\quad\alpha_2+3\alpha_1,
$$
so $\alpha_2$ can be raised three times by $\alpha_1$.