#include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>
#include <fstream>
#include <cassert>
using namespace std;

//common

typedef int  i32;
typedef long long i64,ll;
typedef long double ld;

#define BR "\n"
#define ALL(c) (c).begin(),(c).end()
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define FOR(i,l,r) for(int i=(int)l;i<(int)(r);++i)
#define EACH(it,o) for(auto it = (o).begin(); it != (o).end(); ++it)
#define IN(l,v,r) ((l)<=(v) && (v)<(r))

//config
//#define NDEBUG
//#define INF 1<<30
//#define EPS 1e-8
//const ll MOD =100000007;

//debug
#ifdef NDEBUG
#define DUMP(x)
#define DUMPLN(x)
#define DEBUG(x)
#define DEBUGLN(x)
#define LINE()
#define LINELN()
#define CHECK(exp,act)
#define STOP(e)
#else
#define DUMP(x)  cerr << #x << " = " << (x)
#define DUMPLN(x)  DUMP(x) <<endl
#define DEBUG(x) DUMP(x) << LINE() << " " << __FILE__
#define DEBUGLN(x) DEBUG(x)<<endl
#define LINE()    cerr<< " (L" << __LINE__ << ")"
#define LINELN()    LINE()<<endl
#define CHECK(exp,act)  if(exp!=act){DUMPLN(exp);DEBUGLN(act);}
#define STOP(e)  CHECK(e,true);if(!(e)) exit(1);
#endif

template<class T> inline string toString(const vector<T>& x) {
	stringstream ss;
	REP(i,x.size()){
		if(i!=0)ss<<" ";
		ss<< x[i];
	}
	return ss.str();
}

template<class T> inline string toString(const vector<vector<T> >& map) {
	stringstream ss;
	REP(i,map.size()){
		if(i!=0)ss<<BR;
		ss<< toString(map[i]);
	}
	return ss.str();
}
template<class K,class V>  string toString(map<K,V>& x) {
	string res;stringstream ss;
	for(auto& p:x)ss<< p.first<<":" << p.second<<" ";
	return ss.str();
}

string BITtoString(int bit){
    stringstream ss;
    while(bit!=0){ss<<(bit%2);bit/=2;}
    string res=ss.str();reverse(ALL(res));
    return res;
}

template<typename T,typename V> inline T pmod(T v,V MOD){
	return (v%MOD+MOD)%MOD;
}
#define nextInt(n) scanf("%d",&n)
#define defInt(n) int n;nextInt(n)
#define nextLong(n) scanf("%lld",&n)
#define defLong(n) ll n;nextLong(n)
#define nextDouble(n) scanf("%lf",&n)

namespace ShortestPath{
    typedef double Cost;
    const Cost CINF=1e17;
    typedef  vector<vector<Cost> > Mat;
    struct Edge{
        int to;Cost cost;
        Edge(int to,Cost cost)
            : to(to),cost(cost){};
    };

    typedef vector<vector<Edge> > Graph;

    struct Task{
        int pos;Cost cost;
        Task(int pos,Cost cost)
            :pos(pos),cost(cost){};
        bool operator>(const Task& r) const{
            return cost>r.cost;
        }
    };

      inline vector<Cost> dijkstra(const Graph& g,const int s,vector<int>& prev){
        const int V=g.size();
        vector<Cost> d(V,CINF);d[s]=0;
        
        priority_queue<Task,vector<Task>,greater<Task> > que;que.push(Task(s,0));
        vector<bool> passed(V);
        while(!que.empty()){
            Task task=que.top();que.pop();
            //if(prev[task.pos]!=-2)continue;
            if(passed[task.pos])continue;
            //if(d[task.pos]<task.cost)continue;
            passed[task.pos]=true;
            EACH(e,g[task.pos]){
                if(d[e->to]>d[task.pos]+e->cost){
                    d[e->to]=d[task.pos]+e->cost;
                    que.push(Task(e->to,d[e->to]));
                }
            }   
        }
        return d;
    }
    vector<Cost> dijkstra(const Graph& g,const int s){
        vector<int> prev(g.size());return dijkstra(g,s,prev);
    }
}
using namespace ShortestPath;

class Main{
	public:

	int enc(int pos,int prev,int V){
		return pos*35*N+prev*35+V;
	}
	int N,M;
	
	void run(){
		// ifstream cin("D2");
//		ofstream cout( "D2.out" );
		cout <<fixed<<setprecision(15);	
	
		while(true){
			 // cin >> N >> M;
			 scanf("%d%d",&N,&M);
			if(N==0)break;
			int s,g;
			// cin >> s >> g;
			scanf("%d%d",&s,&g);
			s--;g--;
			
			Graph G(N*N*35);
			REP(i,M){
				int x,y;double d;int c;
				// cin >> x >> y >> d >> c;
				scanf("%d%d%lf%d",&x,&y,&d,&c);
				x--;y--;
				for(int v=0;v<=35;v++)for(int nv=v-1;nv<=v+1;nv++){
					if(!IN(1,nv,c+1))continue;
					REP(prev,N)if(prev!=y)G[enc(x,prev,v)].push_back(Edge(enc(y,x,nv),d/nv));
					REP(prev,N)if(prev!=x)G[enc(y,prev,v)].push_back(Edge(enc(x,y,nv),d/nv));
				}
			}
			vector<Cost> res=dijkstra(G,enc(s,s,0));
			Cost mv=CINF;
			REP(prev,N)mv=min(mv,res[enc(g,prev,1)]);
			if(mv>=CINF) cout <<"unreachable"<<endl;
			else cout << mv<<endl;
		}
	}
};
int main(){
	cout <<fixed<<setprecision(15);	
	ios::sync_with_stdio(false);
	Main().run();
	return 0;
}

Measured memory_bytes: 59236