#include <bits/stdc++.h>
#include <unistd.h>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
 
 
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx[4] = { -1,0, 1,0 }, dy[4] = { 0,1,0,-1 };
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
// #define mp make_pair
//#define endl '\n'

double solve(int n, vii &G, pii kin) {
  vector<double> dp(n);
  dp[n - 1] = 0;
  rrep (i, n - 1) {
    rep (j, G[i].size()) {
      int v = G[i][j];
      if (i == kin.first and v == kin.second) continue;
      dp[i] += dp[v] + 1;
    }
    if (i != kin.first) {
      dp[i] /= (int)(G[i].size());
    }
    else {
      dp[i] /= ((int)(G[i].size()) - 1);
    }
  }
  return dp[0];
}

signed main() {
  fio();

  int n, m;
  cin >> n >> m;
  vii G(n);
  rep (i, m) {
    int u, v;
    cin >> u >> v;
    u--; v--;
    G[u].push_back(v);
  }
  //DEBUG_MAT(G);

  vector<double> dp(n);
  dp[n - 1] = 0;
  rrep (i, n - 1) {
    rep (j, G[i].size()) {
      int v = G[i][j];
      dp[i] += dp[v] + 1;
    }
    dp[i] /= (int)(G[i].size());
  }
  //DEBUG_VEC(dp);
  
  vector<pii> kouho;
  rep (i, n) {
    if (G[i].size() <= 1) continue;
    double ma = -INF, idx = -1;
    rep (j, G[i].size()) {
      int v = G[i][j];
      if (ma < dp[v]) {
        ma = dp[v];
        idx = v;
      }
    }
    kouho.push_back(pii(i, idx));
  }
  //DEBUG_VEC(kouho);

  double ans = dp[0];
  rep (i, kouho.size()) {
    chmin(ans, solve(n, G, kouho[i]));
  }
  Sp(ans);
}


Measured memory_bytes: 2560