#include <bits/stdc++.h>
using namespace std;

// template {{{
#define  range(i, l, r) for (int i = (int)(l); i < (int)(r); (i) += 1)
#define rrange(i, l, r) for (int i = (int)(r) - 1; i >= (int)(l); (i) -= 1)

#define  whole(f, x, ...) ([&](decltype((x)) container) { return (f)(  begin(container),  end(container), ## __VA_ARGS__); })(x)
#define rwhole(f, x, ...) ([&](decltype((x)) container) { return (f)( rbegin(container), rend(container), ## __VA_ARGS__); })(x)

#define debug(x) cerr << "(" << __LINE__ << ")" << #x << ": " << (x) << endl

using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;

// constexpr i32 mod   = 998244353;
constexpr i32 mod   = 1e9 + 7;
constexpr i32 inf   = 1001001001;
constexpr i64 infll = 1001001001001001001ll;

constexpr int dx[] = {0, -1, 1, 0, -1, 1, -1, 1};
constexpr int dy[] = {-1, 0, 0, 1, -1, -1, 1, 1};

struct IoSetup { IoSetup(int x = 15){ cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(x); cerr << fixed << setprecision(x); } } iosetup;

template <typename T = i64> T input() { T x; cin >> x; return x; }

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) { range(i, 0, v.size()) { os << v[i] << (i + 1 != v.size() ? " " : ""); } return os; }
template <typename T> istream &operator>>(istream &is, vector<T> &v) { for (T &in : v) is >> in; return is; }

template <typename T1, typename T2> ostream &operator<<(ostream &os, pair<T1, T2> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
template <typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }

template <typename T> vector<T> make_vector(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_vector(size_t a, Ts... ts) { return vector<decltype(make_vector(ts...))>(a, make_vector(ts...)); }

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
// }}}

void solver() {
    struct edge {
        int to, length, ceil;

        edge() {}
        edge(int to, int length, int ceil) : to(to), length(length), ceil(ceil) {}
    };

    int n, m;
    
    while (cin >> n >> m, n) {
        int s, g;
        cin >> s >> g;

        auto G = make_vector(n + 1, 0, edge());
        range(i, 0, m) {
            int x, y, d, c;
            cin >> x >> y >> d >> c;

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

        auto dist = make_vector(31, 31, 31, double(inf));

        //                    dist                a -> b      v
        using pqtype = pair< double, pair< pair< int, int >, int > >;
        priority_queue<pqtype, vector<pqtype>, greater<pqtype>> pq;
        pq.emplace(0, make_pair(make_pair(0, s), 0));
        dist[0][s][0] = 0;

        while (!pq.empty()) {
            auto p = pq.top();
            pq.pop();

            double fst;
            pair< pair< int, int >, int > snd;

            tie(fst, snd) = p;
            int a, b, v;
            pair< int, int > pp;
            tie(pp, v) = snd;
            tie(a, b) = pp;

            if (dist[a][b][v] < fst) continue;

            for (auto e: G[b]) {
                if (e.to == a) continue;
                auto able = [&](int v) {
                    return v > 0 && v <= e.ceil;
                };

                range(nv, v - 1, v + 2) {
                    if (able(nv) && dist[b][e.to][nv] > fst + (double(e.length) / (nv))) {
                        dist[b][e.to][nv] = fst + (double(e.length) / (nv));
                        pq.emplace(dist[b][e.to][nv], make_pair(make_pair(b, e.to), nv));
                    }
                }
            }
        }

        double ans = inf;
        range(a, 0, 31) chmin(ans, dist[a][g][1]);

        if (ans + 1 > inf) {
            cout << "unreachable" << endl;
        } else {
            cout << ans << endl;
        }
    }
}

signed main(int argc, char *argv[]) {
    solver();
}



Measured memory_bytes: 3888