// OB���h2009 Day3 G : Strange Couple

#include <iostream>
#include <vector>
#include <queue>
#include <cmath>
#include <cstring>

using namespace std;

class Edge{
public:
	int p, q, cost;
	Edge(int p, int q, int cost) : p(p), q(q), cost(cost) {}
	bool operator < (const Edge &e) const { return cost > e.cost; }
};

int main(){
	int n, s, t; 
	while(cin >> n >> s >> t, n){
		s--; t--;
		int cst[100][100];
		int sign[100], out[100];
		for(int i=0;i<n;i++) cin >> sign[i];
		vector< vector<Edge> > graph(n);
		memset(out,0,sizeof(out));
		for(int i=0;i<n;i++){
			for(int j=0;j<n;j++){
				cin >> cst[i][j];
				if(cst[i][j]){ out[i]++; graph[i].push_back(Edge(i,j,cst[i][j])); }
			}
		}
		vector<int> dist(n,-1);
		priority_queue<Edge> qu; qu.push(Edge(-2,t,0));
		while(!qu.empty()){
			Edge e = qu.top(); qu.pop();
			int pos = e.q, ccost = e.cost;
			if(!(dist[pos]<0)) continue;
			dist[pos] = ccost;
			for(int i=0;i<graph[pos].size();i++)
				qu.push(Edge(pos,graph[pos][i].q,ccost+cst[pos][graph[pos][i].q]));
		}
		if(dist[s]==-1) { puts("impossible"); return 0; }
		double a[100][100], b[100];
		for(int i=0;i<n;i++){
			for(int j=0;j<n;j++) a[i][j] = 0;
			b[i] = 0;
		}
		for(int i=0;i<n;i++){
			if(i==t){
				a[i][i] = 1.0; 
			} else if(sign[i]){
				a[i][i] = 1;
				int cnt = 0;
				for(int j=0;j<n;j++){
					if(cst[i][j]!=0&&dist[i]>-1&&dist[j]>-1&&dist[i]==dist[j]+cst[i][j]) cnt++;
				}
				for(int j=0;j<n;j++){
					if(cst[i][j]!=0&&dist[i]>-1&&dist[j]>-1&&dist[i]==dist[j]+cst[i][j]){
						a[i][j] -= 1.0/cnt;
						b[i] += (double)cst[i][j]/cnt;
					}
				}	
			} else {
				a[i][i] = 1;
				for(int j=0;j<n;j++){
					if(cst[i][j]){
						a[i][j] -= 1.0/out[i];
						b[i] += (double)cst[i][j]/out[i];
					}
				}
			}
		}
		for(int i=0;i<n;i++){
			int pivot = i;
			for(int j=i+1;j<n;j++)
				if(abs(a[j][i])>abs(a[pivot][i])) pivot = j;
			for(int j=0;j<n;j++) swap(a[i][j],a[pivot][j]);
			swap(b[i], b[pivot]);
			double div = a[i][i];
			for(int j=0;j<n;j++) a[i][j] /= div; b[i] /= div;
			for(int j=i+1;j<n;j++){
				double mul = a[j][i];
				for(int k=0;k<n;k++)
					a[j][k] -= a[i][k]*mul;
				b[j] -= b[i]*mul;
			}
		}
		for(int i=n-1;i>=0;i--){
			for(int j=0;j<i;j++){
				b[j] -= a[j][i]*b[i];
			}
		}
		double ans = 0.0;
		int cnt = 0;
		for(int i=0;i<n;i++){
			if(dist[i]!=-1){
				cnt++;
				ans += b[i];
			}
		}
		printf("%.9lf\n", b[s]);
	}
}