Problem: 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 don’t 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, t ≤ n, s ≠ t); qi (for 1 ≤ i ≤ n) 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, j ≤ n) 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, j ≤ n. 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.

Example

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

8.50000000

Solution (in CPP):

Solution:
#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;
}