A loop must be drawn.
Draw a single closed loop through all cells of the grid. The loop must make a turn at every disk, and exactly one turn between each pair of disks along the loop.
In this example, we can start by observing that two adjacent cells with disks in them can never be connected. This means that the loop must enter and leave the top cell from the bottom left and bottom right. Then the right end of the loop must make a turn in the next cell, because going straight would end at another disk without having made a turn. If it turns down, then the disk to the right doesn't have enough exists left, so it must turn to the bottom left.
