Even point: the number of line segments from this point is even.

A stroke can have 0 odd points (that is, in a painting, there are no odd points, but all are even points) or 2 odd points.

The problem of a stroke is to judge the number of singularities. If it is 0 or 2, it can be done in one go. If it is greater than 2, it will not work. It can also be popularized. For example, if the number of odd dots is 4, you need 2 strokes. For 6, you need 3 strokes. If there is a singularity, you must start from the singularity.

Extended data

Mathematician Euler found that the law of strokes is:

1. Any connected graph composed of even points can be drawn with one stroke. When drawing, you can start from any even point, and finally you can finish drawing with this point as the end point.

2. Any connected graph with only two singularities (the rest are even points) can be drawn with one stroke. When drawing, one singularity must be the starting point and the other singularity must be the end point.

3. I can't draw any other paintings. (There is even a singularity divided by two, which can be calculated. It takes at least a few strokes to draw this picture. )