18. As shown in the figure, the lengths of the base edges of the rectangular prism are $1 \mathrm{~cm}$ and $3 \mathrm{~cm}$, and the height is $6 \mathrm{~cm}$. If a thin string is used to start from point $A$, pass through the four sides and wrap around once to reach point $B$, then the shortest length of the string used is $\qquad$ $\mathrm{cm}$.
answer: 10