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;

class Node {
  public:
    vector<int> edge;
    Node() {
    }
};

int main()
{
  int nnode, nedge;
  cin >> nnode >> nedge;
  vector<vector<int>> graph(nnode);
  for (int i = 0; i < nedge; i++) {
    int from, to;
    cin >> from >> to;
    from--;
    to--;
    graph.at(from).push_back(to);
  }

  double res = 1e9;
  for (int block = 0; block < nnode; block++) { // when block == nnode - 1, no removal
    vector<double> v(nnode);
    v.back() = 0;
    for (int i = nnode-2; 0 <= i; i--) {
      if (i == block) {
        double sum = 0;
        double maxi = 0;
        for (int to : graph.at(i)) {
          sum += v.at(to);
          maxi = max(maxi, v.at(to));
        }
        v.at(i) = (sum - maxi) / (graph.at(i).size() - 1) + 1;
      } else {
        double sum = 0;
        for (int to : graph.at(i)) {
          sum += v.at(to);
        }
        v.at(i) = sum / graph.at(i).size() + 1;
      }
    }
    res = min(res, v.front());
  }
  printf("%.10lf\n", res);
  return 0;
}