// #define DEBUGGING
#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ALL(V) (V).begin(), (V).end()
#define ALLR(V) (V).rbegin(), (V).rend()

template <typename T> using V = vector<T>;
template <typename T> using VV = V<V<T>>;
using ll = int64_t;
using ull = uint64_t;
using PLL = pair<ll, ll>;

template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return max(low, min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { t = clamp(t, low, high); }

namespace init__ {

struct InitIO {
    InitIO() {
        cin.tie(nullptr);
        ios_base::sync_with_stdio(false);
        cout << fixed << setprecision(30);
    }
} init_io;

}

#ifdef DEBUGGING
#include "../../debug/debug.cpp"
#else
#define DEBUG(...) 0
#define DEBUG_SEPARATOR_LINE 0
#endif

template <typename T>
T make_v(T init) { return init; }

template <typename T, typename... Tail>
auto make_v(T init, size_t s, Tail... tail) {
#define rec make_v(init, tail...)
    return V<decltype(rec)>(s, rec);
#undef rec
}

vector<ll> top_sort(const vector<vector<ll>> &edges) {
    ll N = edges.size();
	vector<ll> ret;
	vector<bool> road(N);
	vector<bool> used(N);
	function<bool(ll)> dfs = [&](ll now) {
		used[now] = true;
		road[now] = true;
		for(ll nxt : edges[now]) {
			if(road[nxt]) return false;
			if(used[nxt]) continue;
			if(!dfs(nxt)) return false;
		}
		ret.push_back(now);
		road[now] = false;
		return true;
	};
	for(ll i = 0; i < edges.size(); i++) {
		if(used[i]) continue;
		if(!dfs(i)) return vector<ll>(0);
	}
	reverse(ret.begin(), ret.end());
	return ret;
}

const double inf = 5e15;

V<double> memo1, memo2;

double dfs(ll cur, const VV<ll> &edges, V<double> &memo) {
    if(memo[cur] != -inf) return memo[cur];
    memo[cur] = 0;
    ll cnt = edges[cur].size();
    for(ll nxt : edges[cur]) memo[cur] += 1 + dfs(nxt, edges, memo);
    memo[cur] /= cnt;
    return memo[cur];
}

int main() {
    ll N, M;
    cin >> N >> M;
    VV<ll> edges(N), redges(N);
    for(ll i = 0; i < M; i++) {
        ll a, b;
        cin >> a >> b;
        a--; b--;
        edges[a].push_back(b);
        redges[b].push_back(a);
    }

    memo1.resize(N, -inf);
    memo2.resize(N, 0);
    memo2[0] = 1;
    for(ll i = 0; i < N; i++) for(ll nxt : edges[i]) memo2[nxt] += memo2[i] / edges[i].size();

    memo1[N - 1] = 0;

    dfs(0, edges, memo1);

    DEBUG(memo1, memo2);

    double ans = memo1[0];
    for(ll from = 0; from < N; from++) {
        if(edges[from].size() <= 1) continue;
        if(redges[from].size() == 0 && from) continue;
        ll cnt = edges[from].size();
        if(cnt == 1) continue;
        for(ll to : edges[from]) {
            double tmp = memo1[from] * cnt - (memo1[to] + 1);
            tmp /= cnt - 1;
            DEBUG(make_tuple(from, to, tmp, memo1[from], memo2[from]));
            DEBUG(memo1[0] + memo2[from] * (tmp - memo1[from]));
            chmin(ans, memo1[0] + memo2[from] * (tmp - memo1[from]));
        }
    }

    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 4864