Problem: There is a cave consisting of N rooms and M one-directional passages. The rooms are numbered 1 through N.

Takahashi is now in Room 1, and Room N has the exit. The i-th passage connects Room s_i and Room t_i (s_i < t_i) and can only be traversed in the direction from Room s_i to Room t_i. It is known that, for each room except Room N, there is at least one passage going from that room.

Takahashi will escape from the cave. Each time he reaches a room (assume that he has reached Room 1 at the beginning), he will choose a passage uniformly at random from the ones going from that room and take that passage.

Aoki, a friend of Takahashi's, can block one of the passages (or do nothing) before Takahashi leaves Room 1. However, it is not allowed to block a passage so that Takahashi is potentially unable to reach Room N.

Let E be the expected number of passages Takahashi takes before he reaches Room N. Find the value of E when Aoki makes a choice that minimizes E.

Constraints

* 2 \leq N \leq 600
* N-1 \leq M \leq \frac{N(N-1)}{2}
* s_i < t_i
* If i != j, (s_i, t_i) \neq (s_j, t_j). (Added 21:23 JST)
* For every v = 1, 2, ..., N-1, there exists i such that v = s_i.

Input

Input is given from Standard Input in the following format:


N M
s_1 t_1
:
s_M t_M


Output

Print the value of E when Aoki makes a choice that minimizes E. Your output will be judged as correct when the absolute or relative error from the judge's output is at most 10^{-6}.

Examples

Input

4 6
1 4
2 3
1 3
1 2
3 4
2 4


Output

1.5000000000


Input

3 2
1 2
2 3


Output

2.0000000000


Input

10 33
3 7
5 10
8 9
1 10
4 6
2 5
1 7
6 10
1 4
1 3
8 10
1 5
2 6
6 9
5 6
5 8
3 6
4 8
2 7
2 9
6 7
1 2
5 9
6 8
9 10
3 9
7 8
4 5
2 10
5 7
3 5
4 7
4 9


Output

3.0133333333

Solution (in CPP):

Solution:
#include "bits/stdc++.h"

using namespace std;
using ll = long long;
using P = pair<int, int>;
#define all(c) (c).begin(), (c).end()
const int INF = 1e9 + 1;
const ll LINF = 1e18 + 1;
const ll MOD = 1e9 + 7;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n, m;
  cin >> n >> m;
  vector<vector<int>> G(n);
  vector<vector<double>> E(n, vector<double>(n, 0.0));

  for (int i = 0; i < m; ++i) {
    int s, t;
    cin >> s >> t;
    --s;
    --t;
    G[s].push_back(t);
  }

  for (int i = n - 2; i >= 0; --i) {
    double sz = G[i].size();
    for (int j = 0; j <= n - 1; ++j) {
      double sum = 0.0, mx = 0.0;
      for (auto t : G[i]) {
        sum += E[t][j] + 1.0;
        mx = max(mx, E[t][j] + 1.0);
      }
      if (i == j) {
        E[i][j] = sz == 1.0 ? sum : (sum - mx) / (sz - 1.0);
      } else {
        E[i][j] = sum / sz;
      }
    }
  }
  // dump(E);
  cout << setprecision(10) << *min_element(all(E[0])) << endl;
  return 0;
}