## Driving Range

**driving.X**

These days, many carmakers are developing cars that run on electricity
instead of gasoline. The batteries used in these cars are generally
very heavy and expensive, so designers must make an important tradeoffs
when determining the battery capacity, and therefore the range, of these
vehicles. Your task is to help determine the minimum range necessary
so that it is possible for the car to travel between any two cities on
the continent.
The road network on the continent consists of cities connected by
bidirectional roads of different lengths. Each city contains a charging station.
Along a route between two cities, the car may pass through any
number of cities, but the distance between each pair of consecutive
cities along the route must be no longer than the range of the car.
What is the minimum range of the car so that there is a route
satisfying this constraint between every pair of cities on the continent?

### Input Specification

The input consists of a sequence of road networks.
The first line of each road network contains two positive integers
*n* and *m*, the number of cities and roads.
Each of these integers is no larger than one million.
The cities are numbered from 0 to *n*-1.
The first line is followed by *m* more lines,
each describing a road. Each such line contains three non-negative
integers. The first two integers are the numbers of the two cities
connected by the road. The third integer is the length of the road.
The last road network is followed by a line containing two zeros,
indicating the end of the input.
### Sample Input

3 3
0 1 3
1 2 4
2 1 5
2 0
0 0

### Output Specification

For each road network, output a line containing one integer,
the minimum range of the car that enables it to drive from every
city to every other city. If it is not possible to drive from
some city to some other city regardless of the range of the
car, instead output a line containing the word `IMPOSSIBLE`.
### Output for Sample Input

4
IMPOSSIBLE

*Ondřej Lhoták*