Xin Li, a poet in the Tang Dynasty, said in the first two sentences of an ancient war song: "mountains cover the white sun, raise your fire and look up to the sky, and drink horses at the boundary river at dusk." There is an interesting math problem in this poem.

As shown in the picture, the general in the poem starts from point A at the foot of the mountain after watching the bonfire, goes to the river to drink horses and then camps at point B. How can the total distance be shortest?

This problem existed as early as ancient Rome. It is said that there is a scholar in Alexandria who is proficient in mathematics and physics, and his name is Helen. One day, a Roman general paid a special visit to him and asked him a puzzling question.

Every day, the general starts from barracks A, goes to the river to drink horses, and then goes to B on the same side of the river for a meeting. How should he walk to get to the shortest distance?

Since then, this problem known as "the general drinks horses" has been widely circulated.

It is not difficult to solve this problem. It is said that Helen solved it after a little thinking.