#include <vector>
#include <list>
#include <map>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <queue>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream>
#include <cstdio>
#include <climits>
#include <complex>
#include <cstdint>
#include <tuple>

#define M_PI       3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util

//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()


struct Edge {
    int to, dist, c;
    Edge(int to, int dist, int c): to(to), dist(dist), c(c) {}
};

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vector<Edge>> AdjList;
AdjList graph;
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
const int INF = 100000000;

ll GCD(ll a, ll b){
    if(a < b) swap(a, b);
    if(b == 0) return a;
    return GCD(b, a%b);
}

bool cmp(const pii& a, const pii& b){
    if(a.first == b.first){
        return a.second < b.second;
    }
    return a.first < b.first;
}

bool prime[1000001];
int mas[1001][1001];
int dp1[1001][1001], dp2[1001][1001];
int dx[4] = { -1, 0, 1, 0 };
int dy[4] = { 0, 1, 0, -1 };

double d[35][35][35];

struct S {
    double time;
    int speed, nowE, prevE;
    
    S(double time, int speed, int nowE, int prevE): time(time), speed(speed), nowE(nowE), prevE(prevE) {}
    
    bool operator > (const S &s) const {
        return time > s.time;
    }
};
int main() {
    cout << fixed << setprecision(6);
    
    int n, m; 
    while(cin >> n >> m){
        if(n == 0 && m == 0 ) break;
        vector<vector<Edge>> G(35);
        int s, g; cin >> s >> g;
        s--;  g--;
        for(int i=0; i<m; ++i){
            int x, y, d, c; cin >> x >> y >> d >> c; x--; y--;
            G[x].push_back(Edge(y, d, c));
            G[y].push_back(Edge(x, d, c));
        }
        
        for(int i=0; i<35; ++i){
            for(int j=0; j<35; ++j){
                for(int k=0; k<35; ++k){
                    d[i][j][k]=1000000.0;
                }
            }
        }
        
        priority_queue<S, vector<S>, greater<S>> q;
        q.push(S(0, 0, s, s));
        // スピード/現在地点/先ほどまでいた地点
        bool visited[35][35][35] = {false};
        d[s][0][s] = 0;
        
        
        
        while(!q.empty()){
            
            S s = q.top();
            q.pop();
            
            double time = s.time;
            int v = s.nowE;
            int speed = s.speed;
            int prev_v = s.prevE;
            
            if(visited[speed][v][prev_v]) continue;
            visited[speed][v][prev_v] = true;
            
            for(int i=0; i<(int)G[v].size(); ++i){
                if(G[v][i].to == prev_v) continue;
                
                Edge e = G[v][i];
                int u = e.to;
                
                int dist = e.dist;
                int c = e.c;
                for(int j=-1; j<=1; ++j){
                    int nsp = speed + j;
                    if(nsp>=1 && nsp<=c && visited[nsp][u][v] == false){
                        if(d[u][nsp][v] > time + (double)dist/nsp){
                            d[u][nsp][v] = time + (double)dist/nsp;
                            q.push(S(d[u][nsp][v], nsp, u, v));
                        }
                    }
                }
            }
        }
        
        double ans = 1000000.0;
        for(int i=0; i<n; ++i){
            ans = min(ans, d[g][1][i]);
        }
        if(ans == 1000000.0) cout << "unreachable" << endl;
        else cout << ans << endl;
    }
    
    return 0;
}


Measured memory_bytes: 3992