Assuming that A walks fast and B walks slowly, then when the two ships meet for the first time, they add up to the width of a river, and B walks slowly, so B, who is closer to the shore, walks 500 meters;

When we met for the second time, both boats reached the shore, so we walked the width of three rivers. Because we walked the width of a river, B walked 500 meters, we walked the width of three rivers, and B walked 500*3= 1500 meters.

At this time, it is on the other side of the river, with a distance of 100 meters. In this process, B takes a river section and adds 100m, so this river section is =1500-100 =1400m.