A problem solved by Euler in about 1736 for the inhabitants of Königsberg (now Kaliningrad). Two islands in a river are connected to each other by one bridge and to the banks by six other bridges; one island has two bridges from the left bank and two from the right bank while the other island has one bridge from the left bank and one from the right bank. The problem is whether or not it is possible to follow a circular walk starting and finishing at the same river bank and crossing each bridge precisely once. See Euler cycle.