#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cstring>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <numeric>
#include <utility>
#include <iomanip>
#include <algorithm>
#include <functional>
using namespace std;

typedef long long ll;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef pair<int,int> pint;
typedef pair<ll,ll> pll;

#define FI first
#define SE second
#define PB push_back
#define MP make_pair
#define ALL(s) (s).begin(),(s).end()
#define REP(i,n) for (int i = 0; i < (int)(n); ++i)
#define EACH(i,s) for (__typeof__((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define COUT(x) cout<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<<endl

template<class T1, class T2> ostream& operator<<(ostream &s, pair<T1,T2> P){return s<<'<'<<P.first<<", "<<P.second<<'>';}
template<class T> ostream& operator<<(ostream &s, vector<T> P) {s<<"{ ";for(int i=0;i<P.size();++i){if(i>0)s<<", ";s<<P[i];}return s<<" }"<<endl;}
template<class T1, class T2> ostream& operator<<(ostream &s, map<T1,T2> P) {s<<"{ ";for(__typeof__(P.begin()) it=P.begin();it!=P.end();++it){if(it!=P.begin())s<<", ";s<<'<'<<it->first<<"->"<<it->second<<'>';}return s<<" }"<<endl;}



template<class T> struct Matrix {
    vector<vector<T> > val;
    Matrix(int n = 1, int m = 1) {val.clear(); val.resize(n, vector<T>(m));}
    Matrix(int n, int m, T x) {val.clear(); val.resize(n, vector<T>(m, x));}
    void resize(int n, int m, T x = 0) {val.clear(); val.resize(n, vector<T>(m));}
    int size() {return val.size();}
    inline vector<T>& operator [] (int i) {return val[i];}
    friend ostream& operator << (ostream& s, Matrix<T> M) {s << M.val; return s;}
};

const double EPS = 1e-10;
template<class T> vector<vector<T> > Gauss_Jordan(Matrix<T> A, vector<T> b) {
	int rank = 0;
    Matrix<T> M(A.size(), A[0].size()+1);
    for (int i = 0; i < A.size(); ++i) {
        for (int j = 0; j < A[0].size(); ++j) M[i][j] = A[i][j];
        M[i][A[0].size()] = b[i];
    }
    
    vint core, rem;
	for (int i = 0; i < A[0].size(); ++i) {
		int pivot = rank;
		T max = 0;
		for (int j = rank; j < M.size(); ++j) {
			if (max < abs(M[j][i])) {
				pivot = j;
				max = abs(M[j][i]);
			}
		}
		if (max > EPS) {
            core.PB(i);
			swap(M[pivot], M[rank]);
            T div = M[rank][i];
            for (int j = 0; j < M[0].size(); ++j) {
                M[rank][j] = M[rank][j] / div;
            }
			for (int j = 0; j < M.size(); ++j) {
				if (j != rank && abs(M[j][i]) > EPS) {
                    T fac = M[j][i];
					for (int k = 0; k < M[0].size(); ++k) {
                        M[j][k] = M[j][k] - M[rank][k] * fac;
                    }
				}
			}
			++rank;
		}
        else rem.PB(i);
	}
    
    vector<vector<T> > res;
    for (int i = rank; i < M.size(); ++i) 
        if (abs(M[i][M[0].size()-1]) > EPS) return res;     // return -1;
    
    vector<T> sol(A[0].size(), 0);
    for (int i = 0; i < core.size(); ++i) sol[core[i]] = M[i][M[0].size()-1];
    res.PB(sol);
    
    for (int i = 0; i < rem.size(); ++i) {
        vector<T> temp(A[0].size(), 0);
        temp[rem[i]] = 1;
        for (int j = 0; j < core.size(); ++j) temp[core[j]] = M[j][rem[i]] * (-1);
        res.PB(temp);
    }
    
    return res;     // return A[0].size()-rank;
};



const int MAX = 105;
const int INF = 1<<29;

int n, s, t, a;
bool issign[MAX];
int graph[MAX][MAX];
int dist[MAX][MAX];

int main() {
    while (cin >> n >> s >> t) {
        if (n == 0) break;
        
        --s; --t;
        Matrix<double> A(n, n, 0.0);
        vector<double> b(n, 0.0);
        
        for (int i = 0; i < n; ++i) scanf("%d", &issign[i]);
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < n; ++j) {
                if (i != j) dist[i][j] = INF;
                else dist[i][j] = 0;
                
                scanf("%d", &a);
                graph[i][j] = a;
                if (a > 0 && i != j) dist[i][j] = a;
            }
        }
        for (int k = 0; k < n; ++k)
            for (int i = 0; i < n; ++i)
                for (int j = 0; j < n; ++j)
                    dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
        
        
        if (dist[s][t] >= INF) {
            puts("impossible");
            continue;
        }
        
        for (int i = 0; i < n; ++i) {
            if (i == t) {
                A[i][i] = 1.0;
                continue;
            }
            if (dist[i][t] >= INF) continue;
            
            vint alt;
            if (issign[i]) {
                int Min = INF;
                for (int j = 0; j < n; ++j) {
                    if (graph[i][j] == 0) continue;
                    if (Min > graph[i][j] + dist[j][t]) {
                        Min = graph[i][j] + dist[j][t];
                        alt.clear(); alt.PB(j);
                    }
                    else if (Min == graph[i][j] + dist[j][t]) {
                        alt.PB(j);
                    }
                }
            }
            else {
                for (int j = 0; j < n; ++j) {
                    if (graph[i][j] != 0) alt.PB(j);
                }
            }
            //cout << i << " : " << alt;
            
            double m = alt.size();
            double sum = 0.0;
            for (int j = 0; j < alt.size(); ++j) sum += graph[i][alt[j]];
            
            for (int j = 0; j < alt.size(); ++j) A[i][alt[j]] = -1.0;
            A[i][i] += m;
            b[i] = sum;
        }
    
        //COUT(A); COUT(b);
        
        vector<vector<double> > res = Gauss_Jordan(A, b);
        
        //COUT(res);
        
        cout << fixed << setprecision(20) << res[0][s] << endl;
    }
    
    return 0;
}


Measured memory_bytes: 1620