#include <bits/stdc++.h>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
using namespace std;
#ifdef LOCAL_DEV
	void debug_impl() { std::cerr << std::endl; }
	template<typename Head, typename... Tail> void debug_impl(Head head, Tail... tail) { std::cerr << " " << head << (sizeof...(tail) ? "," : ""); debug_impl(tail...); }
	#define debug(...) std::cerr << std::boolalpha << "(" << #__VA_ARGS__ << ") ="; debug_impl(__VA_ARGS__); std::cerr << std::noboolalpha;
#else
	#define debug(...)
#endif
#ifdef LOCAL_TEST
	#define BOOST_STACKTRACE_USE_ADDR2LINE
	#define BOOST_STACKTRACE_ADDR2LINE_LOCATION /usr/local/opt/binutils/bin/addr2line
	#define _GNU_SOURCE
	#include <boost/stacktrace.hpp>
	template<typename T> class dvector : public std::vector<T> {
	public:
		dvector() : std::vector<T>() {}
		explicit dvector(size_t n, const T& value = T()) : std::vector<T>(n, value) {}
		dvector(const std::vector<T>& v) : std::vector<T>(v) {}
		dvector(const std::initializer_list<T> il) : std::vector<T>(il) {}
		dvector(const typename std::vector<T>::iterator first, const typename std::vector<T>::iterator last) : std::vector<T>(first, last) {}
		dvector(const std::string::iterator first, const std::string::iterator last) : std::vector<T>(first, last) {}
		T& operator[](size_t n) {
			try { return this->at(n); } catch (const std::exception& e) {
				std::cerr << boost::stacktrace::stacktrace() << std::endl; return this->at(n);
			}
		}
		const T& operator[](size_t n) const {
			try { return this->at(n); } catch (const std::exception& e) {
				std::cerr << boost::stacktrace::stacktrace() << std::endl; return this->at(n);
			}
		}
	};
	class dbool {
	private:
		bool boolvalue;
	public:
		dbool() : boolvalue(false) {}
		dbool(bool b) : boolvalue(b) {}
		dbool(const dbool &b) : boolvalue(b.boolvalue) {}
		operator bool&() { return boolvalue; }
		operator const bool&() const { return boolvalue; }
	};
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::pair<T1, T2>& p) { return s << "(" << p.first << ", " << p.second << ")"; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const dvector<T>& v) {
		for (int i = 0, len = v.size(); i < len; ++i){ s << v[i]; if (i < len - 1) s << "\t"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const dvector< dvector<T> >& vv) {
		for (int i = 0, len = vv.size(); i < len; ++i){ s << vv[i] << std::endl; } return s; }
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::map<T1, T2>& m) {
		s << "{" << std::endl; for (auto itr = m.begin(); itr != m.end(); ++itr){ s << "\t" << (*itr).first << " : " << (*itr).second << std::endl; } s << "}" << std::endl; return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::set<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}" << std::endl; return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::multiset<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}" << std::endl; return s; }
	#define vector dvector
	#define bool dbool
#endif
//#define int long long
using ll = long long;
using bll = boost::multiprecision::cpp_int;
using bdouble = boost::multiprecision::cpp_dec_float_100;
//constexpr int INF = 1e9;//INT_MAX=(1<<31)-1=2147483647
constexpr ll INF = (ll)1e18;//(1LL<<63)-1=9223372036854775807
constexpr ll MOD = (ll)1e9 + 7;
constexpr double EPS = 1e-9;
constexpr int dx[4]={1,0,-1,0};
constexpr int dy[4]={0,1,0,-1};
#define p(var) std::cout<<var<<std::endl
#define rep(i, n)   for(ll i=0, i##_length=(n); i< i##_length; ++i)
#define repeq(i, n) for(ll i=1, i##_length=(n); i<=i##_length; ++i)
#define all(v) (v).begin(), (v).end()
#define uniq(v) (v).erase(unique((v).begin(), (v).end()), (v).end());
template<typename T> inline void pv(vector<T> &v) { for(ll i=0, N=v.size(); i<N; i++) std::cout << v[i] << " \n"[i==N-1]; }
template<typename T> inline T gcd(T a, T b) { return b ? gcd(b,a%b) : a; }
template<typename T> inline T lcm(T a, T b) { return a / gcd(a,  b) * b; }
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); }

/*-----8<-----template-----8<-----*/

/*-----8<-----library-----8<-----*/

constexpr ll M_MAX = 600*599/2;
ll N, M;
ll s[M_MAX];
ll t[M_MAX];
ll dels=-1,delt=-1;

/*
queue<pair<ll,ll>> que;
void bfs(vector<vector<ll>> &g,map<pair<ll,ll>,ll> &depth){
	while (!que.empty()) {
		pair<ll,ll> v = que.front(); // キューから先頭頂点を取り出す
		que.pop();

		// v から辿れる頂点をすべて調べる
		for (auto nextv : g[v.first]) {
			chmax(depth[{nextv,v.first}],v.second+1);
			que.push({nextv,v.second+1});
		}
	}
}

void dfs(vector<vector<ll>> &g,vector<double> &dp,ll v){
	if(v==N-1){
		return;
	}else if(g[v].size()==1){
		dfs(g,dp,g[v][0]);
	}else{
		double maxval=0.;
		for(ll nextv:g[v]){
			if(chmax(maxval,dp[nextv])){
				dels=v;
				delt=nextv;
			}
		}
	}
}
*/

double updatedp(vector<vector<ll>> &g,vector<double> &dp,ll v){
	debug(v);
	if(v==N-1)return 0.;
	if(dp[v]>-0.1)return dp[v];
	double ans=0.;
	ll gvsize=g[v].size();
	for(ll nextv:g[v]){	
		if(v==dels && nextv==delt){
			gvsize--;
			continue;
		}
		ans+=updatedp(g,dp,nextv);
	}
	if(gvsize==0)return INF;
	return dp[v]=1.+ans/gvsize;
}

void solve() {

	//map<pair<ll,ll>,ll> depth;

  	scanf("%lld%lld", &N, &M);
	rep(i, M) {
		scanf("%lld%lld", s + i, t + i);
		s[i]--;t[i]--;
		//depth[{s[i],t[i]}]=0;
	}
	
	vector<vector<ll>> g(N);//,revg(N);
	rep(i,M){
		g[s[i]].push_back(t[i]);
	}
	debug(g);

	vector<double> dptmp(N,-1.);
	double ans=updatedp(g,dptmp,0);

	vector<pair<ll,ll>> delpath;
	rep(v,N){
		if(g[v].size()==1)continue;
		double maxval=0.;
		ll tmps=-1,tmpt=-1;
		for(ll nextv:g[v]){
			if(chmax(maxval,dptmp[nextv])){
				tmps=v;
				tmpt=nextv;
			}
		}
		delpath.push_back({tmps,tmpt});
	}

	for(pair<ll,ll> path:delpath){
		dels=path.first,delt=path.second;
		vector<double> dp(N,-1.);
		double tmp=updatedp(g,dp,0);
		if(chmin(ans,tmp)){debug(dels,delt);}
	}

	printf("%.20f\n", ans);
}

signed main() {
	solve();
	return 0;
}


Measured memory_bytes: 5376