// Slower C++ solution:
#include <algorithm>

#include <bitset>

#include <cmath>

#include <complex>

#include <cstdio>

#include <cstring>

#include <iostream>

#include <map>

#include <queue>

#include <set>

#include <stack>

#include <string>

#include <vector>



#define rep(i,a) for(int i = 0;i < (a); i++)

#define repi(i,a,b) for(int i = (a); i < (b); i++)

#define repd(i,a,b) for(int i = (a); i >= (b); i--)

#define repit(i,a) for(__typeof((a).begin()) i = (a).begin(); i != (a).end(); i++)

#define all(u) (u).begin(),(u).end()

#define rall(u) (u).rbegin(),(u).rend()

#define UNIQUE(u) (u).erase(unique(all(u)),(u).end())

#define pb push_back

#define mp make_pair

#define INF 1e9

#define EPS 1e-9

#define PI acos(-1.0)



using namespace std;



typedef long long ll;

typedef vector<int> vi;

typedef pair<int,int> pii;



int n, m, s, g;

int dv[] = {-1,0,1};



struct S{

    int cur, prev, vel;

    double dist;

    S(int cur, int prev, int vel, double dist):cur(cur),prev(prev),vel(vel),dist(dist){}

    bool operator>(const S &p) const{

        return dist > p.dist;

    }

};



int main() {

    while(cin >> n >> m, n || m){

        cin >> s >> g;

        s--; g--;

        vector<vector<pii> > path(n,vector<pii> (n,mp(INF,INF)));

        rep(i,m){

            int x, y, d, c;

            cin >> x >> y >> d >> c;

            x--; y--;

            path[x][y] = mp(d,c);

            path[y][x] = mp(d,c);

        }



        double d[32][32][32];

        rep(i,32)rep(j,32)rep(k,32) d[i][j][k] = INF;

//        d[s][s][0] = 0;

        priority_queue<S, vector<S>, greater<S> > que;

        que.push(S(s,s,0,0));

        double ans = -1;

        while(!que.empty()){

            S tmp = que.top(); que.pop();

            int cur = tmp.cur, prev = tmp.prev, vel = tmp.vel;

            double dist = tmp.dist;

            if(dist >= d[cur][prev][vel]) continue;

            d[cur][prev][vel] = dist;

            if(vel == 1 && cur == g) {

                ans = dist;

                break;

            }

            rep(i,n)rep(j,3){

                if(i == prev || path[cur][i].first == INF) continue;

                if(vel + dv[j] > 0 && vel + dv[j] <= path[cur][i].second){

                    que.push(S(i,cur,vel+dv[j],dist + 1.0 * path[cur][i].first / (vel+dv[j])));

                }

            }

        }

        if(ans>=0) printf("%.8f\n",ans);

        else cout << "unreachable\n";

    }

    return 0;

}

// Faster C++ solution for the same problem (measured speedup: 1.139416162588956x):
#include <algorithm>

#include <bitset>

#include <cmath>

#include <complex>

#include <cstdio>

#include <cstring>

#include <iostream>

#include <map>

#include <queue>

#include <set>

#include <stack>

#include <string>

#include <vector>



#define repi(i,a,b) for(int i=(a);i<(b);i++)

#define rep(i,a) repi(i,0,a)

#define repd(i,a,b) for(int i=(a);i>=(b);i--)

#define repit(i,a) for(__typeof((a).begin()) i=(a).begin();i!=(a).end();i++)



#define all(u) (u).begin(),(u).end()

#define rall(u) (u).rbegin(),(u).rend()

#define UNIQUE(u) (u).erase(unique(all(u)),(u).end())



#define pb push_back

#define mp make_pair

#define INF 1e9

#define EPS 1e-9

#define PI acos(-1.0)



using namespace std;



typedef long long ll;

typedef vector<int> vi;



struct edge{ int to, cap, d;};

struct node{

    int p, v, prev; double d;

    node(int p, int v, int prev, double d):p(p),v(v),prev(prev),d(d){}

    bool operator<(const node &n)const{ return d > n.d;}

};

const int dv[] = {-1,0,1};

int done[32][32][32], N, M, s, t;

double dist[32][32][32];

vector<edge> G[32];



void solve(){

    priority_queue<node> q;

    q.push(node(s,0,s,0));

    while(!q.empty()){

        node u = q.top(); q.pop();

        if(done[u.p][u.v][u.prev]) continue;

        if(u.p == t and u.v == 1){

            printf("%.8f\n", u.d);

            return;

        }

        done[u.p][u.v][u.prev] = 1;

        dist[u.p][u.v][u.prev] = u.d;

        rep(i,G[u.p].size()){

            edge &e = G[u.p][i];

            if(e.to == u.prev) continue;

            rep(j,3){

                int nv = u.v + dv[j];

                if(nv > 0 and nv <= e.cap)

                    q.push(node(e.to, nv, u.p, 1.*e.d/nv+u.d));

            }

        }

    }

    puts("unreachable");

}



void input(){

    cin >> s >> t;

    s--; t--;

    rep(i,M){

        int x, y, d, c; cin >> x >> y >> d >> c;

        x--; y--;

        G[x].pb((edge){y,c,d});

        G[y].pb((edge){x,c,d});

    }

}



void init(){

    rep(i,32) G[i].clear();

    memset(done, 0, sizeof(done));

}



int main()

{

    while(cin  >> N >> M, N or M){

        init();

        input();

        solve();

    }

    return 0;

}