Principle: If a graph can be completed in one stroke, it must meet two conditions, namely, the graph is closed connected, and the number of singularities or points connected with odd edges in the graph is 0 or 2.

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. Connected graphs with only two singularities are even points and 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.

Note: If an even number of singularities are divided by two, you can work out how many strokes are needed for this diagram.