7. The figure shows a city plan. Nodes are intersections, and the 35 lines connecting them are streets. There are $N$ buses operating on these streets. All buses start simultaneously at intersections and move to adjacent intersections along the streets every minute. Each bus follows a closed, non-self-intersecting route. Multiple buses can be at the same intersection, but no more than one bus can be on any street at the same time. Find the maximum possible value of $N$.

![](https://cdn.mathpix.com/cropped/2024_05_06_dd0a2f88a6d78770ccfbg-2.jpg?height=303&width=309&top_left_y=114&top_left_x=1659)
Answer: 35 (by the number of streets), i.e., it is possible to launch the minibuses so that at any moment in time, exactly one minibus is moving along each street. Obviously, $N$ cannot be greater than 35, because otherwise, in the first minute, some two minibuses will inevitably end up on the same street. Let's provide an example for 35 minibuses. Seven minibuses will be launched clockwise around the middle ring. The remaining 28 minibuses will move clockwise around the perimeters of seven $2 \times 2$ squares, with four on each, initially positioned at the vertices of these squares. It is not difficult to verify (taking into account the circular symmetry of the construction) that at any moment in time, exactly one minibus is driving along each street.