Highway Tolls

문제

In Japan, cities are connected by a network of highways. This network consists of N cities and M highways. Each highway connects a pair of distinct cities. No two highways connect the same pair of cities. Cities are numbered from 0 through N - 1, and highways are numbered from 0 through M - 1. You can drive on any highway in both directions. You can travel from any city to any other city by using the highways.

A toll is charged for driving on each highway. The toll for a highway depends on the traffic condition on the highway. The traffic is either light or heavy. When the traffic is light, the toll is A yen (Japanese currency). When the traffic is heavy, the toll is B yen. It's guaranteed that A < B. Note that you know the values of A and B.

You have a machine which, given the traffic conditions of all highways, computes the smallest total toll that one has to pay to travel between the pair of cities S and T (S ≠ T), under the specified traffic conditions.

However, the machine is just a prototype. The values of S and T are fixed (i.e., hardcoded in the machine) and not known to you. You would like to determine S and T. In order to do so, you plan to specify several traffic conditions to the machine, and use the toll values that it outputs to deduce S and T. Since specifying the traffic conditions is costly, you don't want to use the machine many times.