Find the trail map of a cross-country ski area. Represent the trail map as an undirected graph, where each intersection of trails is a vertex, and each section of trail between intersections is an edge. Consider a cross-country skier who wishes to take the longest tour possible, but does not want to ski on any trail more than once. What is the longest path that starts and ends at the ski lodge and does not traverse any section of trail more than once? (Intersections may be passed through more than once, and some sections of trail may be left unskied.)