84. As shown in the figure, two people, A and B, are walking along a path consisting of semicircles with diameters equal to the four sides of a square. The arc length of each semicircle is $100 \mathrm{~m}$. A starts from point $A$ and B starts from point $B$, both moving counterclockwise at the same time. A walks at a speed of $120 \mathrm{~m}$ per minute, and B walks at a speed of $150 \mathrm{~m}$ per minute. Each time they pass through points $A, B, C, D$ in the figure, they each spend 1 second. After starting, B catches up with A for the first time after a certain number of seconds.
Answer: 209