『杭电1122』Direct Visibility

Problem Description

Building the GSM network is a very expensive and complex task. Moreover, after the Base Transceiver Stations (BTS) are built and working, we need to perform many various measurements to determine the state of the network, and propose effective improvements to be made.
The ACM technicians have a special equipment for measuring the strength of electro-magnetic fields, the transceivers' power and quality of the signal. This equipment is packed into a huge knapsack and the technician must move with it from one BTS to another. Unfortunately, the knapsack have not enough memory for storing all of the measured values. It has a small cache only, that can store values for several seconds. Then the values must be transmitted to the BTS by an infrared connection (IRDA). The IRDA needs direct visibility between the technician and the BTS.

Your task is to find the path between two neighbouring BTSes