A: Shortest Path
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Given a directed graph, and two nodes, find the shortest path between them. 

Input
-----
There will be multiple instances.  For each instance, the first line
of input will be four integers n, m, s and t (1 <= s,t <= n <= 3000
and 0 <= m <= 1000000) separated by single spaces, where n is the number 
of nodes, and s and t are node indices.  The nodes are numbered 1 to n,
and the indices refer to this numbering scheme.  Following this, there are 
exactly m lines.  Each of the m lines consists of three positive integers 
u, v, w (1 <= u,v <= n and 1 <= w <= 10000 with u not equal to v) 
separated by a single space.  Such a line indicates that there is an 
edge from node number u to node number v with weight w.  There is at most
one edge from each node to each other node.

The end of the input is indicated by a line that has n=m=s=t=0.

Output
------
The minimum total weight of any path from node number s to node number t.
If there is no such path, your programme should output "unreachable".

Sample Input
------------
2 3 1 3
1 2 7
2 3 10
1 3 19
2 1 2 1
1 2 876
0 0 0 0

Sample output
-------------
17
unreachable