23. As shown in Figure 11, a student starts from point $A$, moves forward 1 meter, turns right $18^{\circ}$, then moves forward 1 meter again, turns right $18^{\circ}$, moves forward 1 meter again, and turns right $18^{\circ}$, and so on. When he first returns to the starting point $\mathrm{A}$, he has walked $\qquad$ meters.
Reference answer: 20