The first time: a and b go to the other side, and a comes back to pick up C.

The second time: a and c go to the other side, and a comes back to pick up d.

……

Twelfth time: a and m go to the other side, and a comes back to pick up n.

Thirteenth time: A and N go to the other side.

A total of * * * crossing the river 13 times