// 2009�‘“à—\‘ID —£ŽU“I‘¬“x

#include <iostream>
#include <vector>
#include <queue>

using namespace std;

typedef vector< vector<int> > graph;

class Node{
public:
	int p, v, prev; double cost;
	Node(int p, int v, int prev, double cost) : p(p), v(v), prev(prev), cost(cost) {}
	bool operator < (const Node &nd) const { return cost > nd.cost; }
};

int main(){
	int n, m, s, g;
	while(cin >> n >> m){
		if(!(n||m)) break;
		cin >> s >> g;
		graph gp(n);
		int dist[30][30];
		int limit[30][30];
		bool visit[30][31][30];
		double len[30][31][30];
		memset(visit, false, sizeof(visit));
		for(int i=0;i<m;i++){
			int x, y, d, c;
			cin >> x >> y >> d >> c;
			gp[x-1].push_back(y-1);
			gp[y-1].push_back(x-1);
			dist[x-1][y-1] = d,  dist[y-1][x-1] = d;
			limit[x-1][y-1] = c, limit[y-1][x-1] = c;
		}
		for(int i=0;i<n;i++)
			for(int j=0;j<=30;j++)
				for(int k=0;k<n;k++) len[i][j][k] = 1e15;
		priority_queue<Node> qu; qu.push(Node(s-1,0,s-1,0.0));
		double ans = -1.0;
		while(!qu.empty()){
			Node nd = qu.top(); qu.pop();
			int p = nd.p, v = nd.v, prev = nd.prev;
			if(visit[p][v][prev]) continue;
			visit[p][v][prev] = true;
			if(p==g-1 && v==1){
				ans = nd.cost;
				break;
			}
			for(int i=0;i<gp[p].size();i++){
				int next = gp[p][i];
				if(next==prev) continue;
				for(int j=-1;j<=1;j++){
					int nv = v+j;
					if(nv<=0||limit[p][next]<nv) continue;
					if(len[next][nv][p]-1e-8 > nd.cost+(double)dist[p][next]/nv){
						len[next][nv][p] = nd.cost+(double)dist[p][next]/nv;
						qu.push(Node(next,nv,p,len[next][nv][p]));
					}
				}
			}
		}
		if(ans < 0.0) cout << "unreachable" << endl;
		else          printf("%.5f\n", ans);
	}
}