In Fig. $3,$ sound waves A and $B,$ both of wavelength $,$ are initially in phase and traveling rightward, as indicated by the two rays. Wave $A$ is reflected from four surfaces but ends up traveling in its original direction. What multiple of wavelength $$ is the smallest value of distance $L$ in the figure that puts A and $B$ exactly out of phase with each other after the reflections? $(10$ points$)$

Figure $3$