//#define DEBUG
#include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define REP(i,n) for(int i=0;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1<<29
#define LINF LLONG_MAX/3
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define ALL(v) (v).begin(),(v).end()
#define all(v) ALL(v)
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
#define CININIT cin.tie(0),ios::sync_with_stdio(false)
struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/
 
// (time,speed,node,pre_node)
using P = pair<pair<double,int>,pair<int,int>>;
 
 
P makeP(double t,int s,int n,int np){
    return make_pair(make_pair(t,s),make_pair(n,np));
}
 
void solve(int n,int m){
    int start,goal; cin>>start>>goal;
    start--;
    goal--;
    vector<vector<int>> g(n,vector<int>(n,INF));
    vector<vector<int>> lim(n,vector<int>(n,0));
    rep(i,m){
        int x,y,d,c;
        cin>>x>>y>>d>>c;
        x--;
        y--;
        g[x][y] = d;
        g[y][x] = d;
        lim[x][y] = c;
        lim[y][x] = c;
    }
 
    double visited[32][32][32];
    rep(i,32) rep(j,32) rep(k,32) visited[i][j][k]=DBL_MAX/10;
    visited[start][0][start] = 0;
 
    priority_queue<P,vector<P>,greater<P>> que;
    que.push(makeP(0,0,start,start));
    while(!que.empty()){
        auto p = que.top();
        que.pop();
        double ti = p.first.first;
        int speed = p.first.second;
        int node = p.second.first;
        int pre = p.second.second;
 
#ifdef DEBUG
        cerr << "time : " << ti << " "  << "node : " << node+1 << " " << "speed " << speed << endl;
#endif
        if(node==goal and speed==1){
            cout << Double(ti) << endl;
            return;
        }
 
        if(ti > visited[node][speed][pre]) continue;
 
        for(int next_node=0;next_node<n;next_node++){
            if(next_node==node or next_node==pre or g[node][next_node]==INF) continue;
            int d=g[node][next_node];
            int c=lim[node][next_node];
            for(int nspeed=speed-1;nspeed<=speed+1;nspeed++){
                if(nspeed<=0 or nspeed>c) continue;
                double nt = ti + (double)d/(double)nspeed;
                if(nt < visited[next_node][nspeed][node]){
#ifdef DEBUG
                    cerr << "\ttime " << nt << " " << "node " << next_node+1 << " speed " << nspeed << endl;
#endif
                    visited[next_node][nspeed][node] = nt;
                    que.push(makeP(nt,nspeed,next_node,node));
                }
            }
        }
    }
    cout << "unreachable" << endl;
}
 
int main(){
    int n,m;
    while(cin>>n>>m){
        if(n==0 && m==0) break;
        solve(n,m);
    }
}

Measured memory_bytes: 3896