On one side of a river, there are three policemen and three criminals.

* There is a boat which can be used to transfer people from one side of the river to the other.

* No more than two people can fit in the boat and it must have at least one person in it during any transfer.

* The problem is to find the shortest sequence of transfers which gets all six people from one side to the other

* without ever creating a situation where criminals outnumber policemen (if they were both) on either side of the river.