#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define srep(i,s,t) for(int i=(int)(s);i<(int)(t);++i)
#define each(a,b) for(auto (a): (b))
#define all(v) (v).begin(),(v).end()
#define len(v) (int)(v).size()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define cmx(x,y) x=max(x,y)
#define cmn(x,y) x=min(x,y)
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define spair(p) cout<<#p<<": "<<p.fi<<" "<<p.se<<endl
#define svec(v) cout<<#v<<":";rep(kbrni,v.size())cout<<" "<<v[kbrni];cout<<endl
#define sset(s) cout<<#s<<":";each(kbrni,s)cout<<" "<<kbrni;cout<<endl
#define smap(m) cout<<#m<<":";each(kbrni,m)cout<<" {"<<kbrni.first<<":"<<kbrni.second<<"}";cout<<endl

using namespace std;

typedef pair<int,int> P;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<double> vd;
typedef vector<P> vp;
typedef vector<string> vs;

const int MAX_N = 100005;

int n,s,t;

template<typename T> class Dijkstra {
public:
	struct edge{
		int to; T cost;
	};
	vector<vector<edge> > G;
	vector<T> d;
	int V;
	using pti = pair<T,int>;
	Dijkstra(int node_size){
		V = node_size;
		G.resize(V),d.resize(V,numeric_limits<T>::max());
	}
	//無向グラフの場合
	void add_edge(int u,int v,T cost){
		G[u].pb((edge){v,cost}),G[v].pb((edge){u,cost});
	}
	void solve(int s){
		priority_queue<pti,vector<pti>,greater<pti> > que;
		d[s] = 0;
		que.push(pti(0,s));
		while(!que.empty()){
			pti p = que.top();
			que.pop();
			int v = p.second;
			if(d[v] < p.first) continue;
			for(auto& w : G[v]){
				if(d[w.to] > d[v] + w.cost){
					d[w.to] = d[v] + w.cost;
					que.push(pti(d[w.to],w.to));
				}
			}
		}
	}
};

template<typename T> class mat : public vector<vector<T> > {
private:
    int r,c;    //行,列
public:
    int row() const {
        return r;
    }
    int column() const {
        return c;
    }
    mat(int n,int m,T val = 0){
        this->r = n,this->c = m;
        rep(i,n){
            this->push_back(vector<T>(m,val));
        }
    }
    mat operator+(const mat& another){
        if(this->r != another.r && this->c != another.c){
            cout << "足し算失敗(サイズ不一致)" << endl;
            exit(1);
        }
        mat<T> X(this->r,this->c);
        rep(i,this->r){
            rep(j,this->c){
                X[i][j] = (*this)[i][j] + another[i][j];
            }
        }
        return X;
    }
    mat operator+(const T val){
        mat<T> X(this->r,this->c);
        rep(i,this->r){
            rep(j,this->c){
                X[i][j] = (*this)[i][j] + val;
            }
        }
        return X;
    }
    mat operator-(const mat& another){
        if(this->r != another.r && this->c != another.c){
            cout << "引き算失敗(サイズ不一致)" << endl;
            exit(1);
        }
        mat<T> X(this->r,this->c);
        rep(i,this->r){
            rep(j,this->c){
                X[i][j] = (*this)[i][j] - another[i][j];
            }
        }
        return X;
    }
    mat operator-(const T val){
        mat<T> X(this->r,this->c);
        rep(i,this->r){
            rep(j,this->c){
                X[i][j] = (*this)[i][j] - val;
            }
        }
        return X;
    }
    vector<T> operator*(const vector<T>& another){
        if(this->c != another.size()){
            cout << "掛け算失敗(サイズ不一致)" << endl;
            exit(1);
        }
        vector<T> vec(this->r,0);
        rep(i,this->r){
            rep(j,this->c){
                vec[i] += (*this)[i][j] * another[j];
            }
        }
        return vec;
    }
    mat operator*(const mat& another){
        if(this->c != another.r){
            cout << "掛け算失敗(サイズ不一致)" << endl;
            exit(1);
        }
        mat<T> X(this->r,another.c);
        rep(i,this->r){
            rep(k,this->c){
                rep(j,another.c){
                    X[i][j] += (*this)[i][k]*another[k][j];
                }
            }
        }
        return X;
    }
    mat operator-(){
        mat<T> X(this->r,this->c);
        rep(i,this->r){
            rep(j,this->c){
                X[i][j] = -(*this)[i][j];
            }
        }
        return X;
    }
    int rank(){
        int n = this->r;
        int res = 0;
        mat B(n,n);
        rep(i,n){
            rep(j,n){
                B[i][j] = (*this)[i][j];
            }
        }
        rep(i,n){
            int pivot = i;
            for(int j=i;j<n;j++){
                if(abs(B[j][i]) > abs(B[pivot][i])){
                    pivot = j;
                }
            }
            if(abs(B[pivot][i]) < EPS){
                continue;
            }
            swap(B[i],B[pivot]);
            for(int j=i+1;j<n;j++){
                B[i][j] /= B[i][i];
            }
            for(int j=i+1;j<n;j++){
                for(int k=i+1;k<n;k++){
                    B[j][k] -= B[j][i] * B[i][k];
                }
            }
            res++;
        }
        return res;
    }
    T det(){
        if(this->r != this->c){
            cout << "正方行列でない(行列式定義不可)" << endl;
            exit(1);
        }
        T ans = 1;
        int n = this->r;
        mat B(n,n);
        rep(i,n){
            rep(j,n){
                B[i][j] = (*this)[i][j];
            }
        }
        rep(i,n) {
            for(int j=i+1;j<n;j++){
                for (; B[j][i] != 0; ans = -ans) {
                    T r = B[i][i] / B[j][i];
                    for(int k=i;k<n;k++) {
                        T t = B[i][k] - r * B[j][k];
                        B[i][k] = B[j][k];
                        B[j][k] = t;
                    }
                }
           }
           ans *= B[i][i];
       }
       return ans;
    }
    mat inverse(){
        if(this->r != this->c){
            cout << "正方行列でない(逆行列定義不可)" << endl;
            exit(1);
        }
        int n = this->r;
        mat B(n,2*n);
        rep(i,n){
            rep(j,n){
                B[i][j] = (*this)[i][j];
            }
        }
        rep(i,n){
            B[i][n+i] = 1;
        }
        rep(i,n){
            int pivot = i;
            for(int j=i;j<n;j++){
                if(abs(B[j][i]) > abs(B[pivot][i])){
                    pivot = j;
                }
            }
            if(abs(B[pivot][i]) < EPS){
                cout << "解なしor不定" << endl;
                exit(1);
            }
            swap(B[i],B[pivot]);
            for(int j=i+1;j<=2*n;j++){
                B[i][j] /= B[i][i];
            }
            rep(j,n){
                if(i != j){
                    for(int k=i+1;k<=2*n;k++){
                        B[j][k] -= B[j][i] * B[i][k];
                    }
                }
            }
        }
        mat res(n,n);
        rep(i,n){
            rep(j,n){
                res[i][j] = B[i][n+j];
            }
        }
        return res;
    }
    void print(){
        rep(i,this->r){
            rep(j,(this->c)-1){
                cout << (*this)[i][j] << ",";
            }
            cout << (*this)[i][(this->c)-1] << endl;
        }
    }
};

template<typename T> vector<T> eq_solve(const mat<T>& A,const vector<T>& b){
    if(A.row() != A.column()){
        cout << "正方行列でない(解なしor不定)" << endl;
        exit(1);
    }
    int n = A.row();
    mat<T> B(n,n+1);
    rep(i,n){
        rep(j,n){
            B[i][j] = A[i][j];
        }
    }
    rep(i,n){
        B[i][n] = b[i];
    }
    rep(i,n){
        int pivot = i;
        for(int j=i;j<n;j++){
            if(abs(B[j][i]) > abs(B[pivot][i])){
                pivot = j;
            }
        }
        if(abs(B[pivot][i]) < EPS){
            cout << "解なしor不定" << endl;
            exit(1);
        }
        swap(B[i],B[pivot]);
        for(int j=i+1;j<=n;j++){
            B[i][j] /= B[i][i];
        }
        rep(j,n){
            if(i != j){
                for(int k=i+1;k<=n;k++){
                    B[j][k] -= B[j][i] * B[i][k];
                }
            }
        }
    }
    vector<T> res(n);
    rep(i,n){
        res[i] = B[i][n];
    }
    return res;
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    while(1){
        cin >> n >> s >> t;
        if(n == 0){
            break;
        }
        --s,--t;
        Dijkstra<int> dj(n);
        vi q(n);
        vvi a(n,vi(n));
        rep(i,n){
            cin >> q[i];
        }
        rep(i,n){
            rep(j,n){
                cin >> a[i][j];
                if(i < j && a[i][j]){
                    dj.add_edge(i,j,a[i][j]);
                }
            }
        }
        dj.solve(t);
        int opt = dj.d[s];
        if(opt == INF){
            cout << "impossible\n";
            continue;
        }
        // show(opt);
        mat<double> A(n,n);
        rep(i,n){
            int sm = 0;
            if(q[i]){
                int mn = INF;
                vi hoge;
                rep(j,n){
                    if(a[i][j]){
                        if(mn > dj.d[j]+a[i][j]){
                            hoge.clear();
                            mn = dj.d[j]+a[i][j];
                            hoge.pb(j);
                        }else if(mn == dj.d[j]+a[i][j]){
                            hoge.pb(j);
                        }
                    }
                }
                if(len(hoge)){
                    rep(j,len(hoge)){
                        A[i][hoge[j]] = 1.0 / len(hoge);
                    }
                }
            }else{
                rep(j,n){
                    if(a[i][j]){
                        A[i][j] = 1;
                        sm++;
                    }
                }
                if(sm){
                    rep(j,n){
                        A[i][j] /= (double)sm;
                    }
                }
            }
        }
        // A.print();
        map<int,int> mp;
        vi unzip(n-1);
        mat<double> B(n-1,n-1);
        vd vec(n-1);
        int id = 0;
        rep(i,n){
            if(i != t){
                unzip[id] = i;
                mp[i] = id++;
            }
        }
        rep(i,n-1){
            int p = unzip[i];
            rep(j,n-1){
                int q = unzip[j];
                B[i][j] -= A[p][q];
                vec[i] += a[p][q]*A[p][q];
            }
            vec[i] += a[p][t]*A[p][t];
            B[i][i] += 1.0;
        }
        // show("\n");
        // B.print();
        // svec(vec);
        vd res = eq_solve(B,vec);
        printf("%.12lf\n",res[mp[s]]);
    }
    return 0;
}



Measured memory_bytes: 3332