#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int INF = (int)1e9;
const long long LINF = (long long)1e18;
const long double PI = acos((long double)-1);
const long double EPS = 1e-9;
inline long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
inline long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
inline int chkmin(T& a, const T& val) {
  return a > val ? a = val, 1 : 0;
}
template <class T>
inline int chkmax(T& a, const T& val) {
  return a < val ? a = val, 1 : 0;
}
inline long long isqrt(long long k) {
  long long r = sqrt(k) + 1;
  while (r * r > k) r--;
  return r;
}
inline long long icbrt(long long k) {
  long long r = cbrt(k) + 1;
  while (r * r * r > k) r--;
  return r;
}
inline void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
inline void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
const int maxn = 410;
int n, m;
vector<int> adj[maxn];
int d[maxn][maxn];
vector<int> g[maxn];
long double p[maxn];
long double q[maxn];
long double r[maxn];
void solve() {
  cin >> n >> m;
  memset(d, 0x3f, sizeof(d));
  for (int i = (0); i < (m); i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    adj[u].push_back(v);
    adj[v].push_back(u);
    d[u][v] = d[v][u] = 1;
  }
  for (int i = (0); i < (n); i++) d[i][i] = 0;
  for (int k = (0); k < (n); k++)
    for (int i = (0); i < (n); i++)
      for (int j = (0); j < (n); j++) chkmin(d[i][j], d[i][k] + d[k][j]);
  long double ans = 0;
  for (int i = (0); i < (n); i++) {
    for (int j = (0); j < (n); j++) g[j].clear();
    for (int j = (0); j < (n); j++) g[d[i][j]].push_back(j);
    long double tot = 0;
    for (int j = (0); j < (n); j++)
      if (int((g[j]).size())) {
        memset(p, 0, sizeof(p));
        vector<int> ver;
        for (int k = (0); k < (int((g[j]).size())); k++) {
          int l = g[j][k];
          for (int u = (0); u < (int((adj[l]).size())); u++) {
            int v = adj[l][u];
            if (p[v] == 0) {
              ver.push_back(v);
            }
            p[v] += 1.0 / int((adj[l]).size());
          }
        }
        long double tmp = 0;
        for (int k = (0); k < (n); k++) {
          for (int l = (0); l < (int((ver).size())); l++) {
            int v = ver[l];
            chkmax(q[d[k][v]], p[v]);
            r[d[k][v]] += p[v];
          }
          long double z = 0;
          for (int l = (0); l < (int((ver).size())); l++) {
            int v = ver[l];
            if (q[d[k][v]] != 0) {
              z += q[d[k][v]];
              q[d[k][v]] = 0;
            }
          }
          chkmax(tmp, z);
        }
        tot += max((long double)1.0, tmp) / n;
      }
    chkmax(ans, tot);
  }
  cout << fixed << setprecision(9) << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  solve();
  return 0;
}
