Strange Couple

Time Limit : 8 sec, Memory Limit : 65536 KB

Problem G: Strange Couple

Alice and Bob are going to drive from their home to a theater for a date. They are very challenging - they have no maps with them even though they donft know the route at all (since they have just moved to their new home). Yes, they will be going just by their feeling.

The town they drive can be considered as an undirected graph with a number of intersections (vertices) and roads (edges). Each intersection may or may not have a sign. On intersections with signs, Alice and Bob will enter the road for the shortest route. When there is more than one such roads, they will go into one of them at random.

On intersections without signs, they will just make a random choice. Each random selection is made with equal probabilities. They can even choose to go back to the road they have just come along, on a random selection.

Calculate the expected distance Alice and Bob will drive before reaching the theater.

Input

The input consists of multiple datasets. Each dataset has the following format:

n s t
q1 q2 ... qn
a11 a12 ... a1n
a21 a22 ... a2n
.
.
.
an1 an2 ... ann

n is the number of intersections (n ≤ 100). s and t are the intersections the home and the theater are located respectively (1 ≤ s, tn, st); qi (for 1 ≤ in) is either 1 or 0, where 1 denotes there is a sign at the i-th intersection and 0 denotes there is not; aij (for 1 ≤ i, jn) is a positive integer denoting the distance of the road connecting the i-th and j-th intersections, or 0 indicating there is no road directly connecting the intersections. The distance of each road does not exceed 10.

Since the graph is undirectional, it holds aij = aji for any 1 ≤ i, jn. There can be roads connecting the same intersection, that is, it does not always hold aii = 0. Also, note that the graph is not always planar.

The last dataset is followed by a line containing three zeros. This line is not a part of any dataset and should not be processed.

Output

For each dataset, print the expected distance accurate to 10-8 , or "impossible" (without quotes) if there is no route to get to the theater. The distance may be printed with any number of digits after the decimal point.

Sample Input

5 1 5
1 0 1 0 0
0 2 1 0 0
2 0 0 1 0
1 0 0 1 0
0 1 1 0 1
0 0 0 1 0
0 0 0

Output for the Sample Input

8.50000000

Source: ACM-ICPC Japan Alumni Group Summer Camp 2009 , Day 3, Tokyo, Japan, 2009-09-20
http://acm-icpc.aitea.net/