On a bus route between towns A and H, there are six other towns: B, C, D, E, F and G. Each segment of the route between two towns that passes through exactly two more towns is of length at least 23 km. Each segment of the route between two towns that passes through exactly one more town is of length at most 18 km. If the length of the whole route is 51 km, what are possible lengths of the segment of the route between D and E?