#include <bits/stdc++.h>
#define loop(n) for (int ngtkana_is_genius = 0; ngtkana_is_genius < int(n); ngtkana_is_genius++)
#define rep(i, begin, end) for(int i = int(begin); i < int(end); i++)
#define all(v) v.begin(), v.end()
#define rand(l, r) std::uniform_int_distribution<>(l, r)(mt)
using lint = long long;
auto mt = std::mt19937_64(std::random_device{}());
auto cmn = [](auto& a, auto b){if (a > b) {a = b; return true;} return false;};
auto cmx = [](auto& a, auto b){if (a < b) {a = b; return true;} return false;};
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail){
  std::cerr << " " << head;
  debug_impl(tail...);
}
#ifdef STOPIT
#define debug(...) 0
#else
#define debug(...)\
do {\
  std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
  debug_impl(__VA_ARGS__);\
  std::cerr << std::noboolalpha;\
} while (false)
#endif

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr>
std::istream& operator>> (std::istream& is, Container& v)
  { for (auto & x : v) { is >> x; } return is; }

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr >
std::ostream& operator<< (std::ostream& os, Container const& v) {
 os << "{";
  for (auto it = v.begin(); it != v.end(); it++)
    {os << (it != v.begin() ? "," : "") << *it;}
  return os << "}";
}

template <typename T>
auto make_vector_impl(size_t sz, T t) {return std::vector<T>(sz, t);}

template <size_t N, typename T, typename U, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz, U u) {return make_vector_impl(sz, T(u));}

template <size_t N, typename T, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz) {return std::vector<T>(sz);}

template <size_t N, typename T, typename... Args, std::enable_if_t<N != 1, std::nullptr_t> = nullptr>
auto make_vector(size_t a, Args... args) {return make_vector_impl(a, make_vector<N - 1, T>(args...));}

template <typename T, typename Size_t>
auto& at(T& t, Size_t i) {return t.at(i);}

template <typename T, typename Size_t, typename... Args>
auto& at(T& t, Size_t i, Args... args) {return at(t.at(i), args...);}

int main() {
  std::cin.tie(0); std::cin.sync_with_stdio(false);
  int n, m; std::cin >> n >> m;
  auto graph = make_vector< 2, int >(n, 0);
  loop(m) {
    int u, v; std::cin >> u >> v;
    u--, v--;
    graph.at(u).emplace_back(v);
  }
  for (auto & v : graph) {
    std::sort(all(v));
    std::reverse(all(v));
  }

  double inf = std::numeric_limits< double >::max();
  auto cal = [&] (int skip) {
    if (skip != -1 && graph.at(skip).size() == 1u) {
      return inf;
    }
    std::vector< double > dp(n, -1);
    dp.at(n - 1) = 0;
    for (int i = n - 2; i >= 0; i--) {
      auto a = graph.at(i);
      std::vector< double > b(a.size());
      std::transform(all(a), b.begin(),
        [&](auto j){ return dp.at(j) + 1; });
      std::sort(all(b));
      if (i == skip) b.pop_back();
      dp.at(i) = std::accumulate(all(b), 0.0) / b.size();
    }
    return dp.at(0);
  };

  double ans = inf;
  rep(i, -1, n - 1) {
    cmn(ans, cal(i));
  }
  std::cout << std::setprecision(10) << ans << std::endl;
  return 0;
}


Measured memory_bytes: 1408