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;
#define inf 1e9

int main(){
  int N, M, i, j;
  scanf("%d%d", &N, &M);
  vector<vector<int>> g(N, vector<int>(0));
  for(i = 0; i < M; i++){
    int s, t;
    scanf("%d%d", &s, &t);
    s--;
    t--;
    g[s].push_back(t);
  }
  double ans = inf;
  for(int x = 0; x < N; x++){
    if(g[x].size() == 1){
      continue;
    }
    vector<double> E(N, 0);
    for(i = N - 2; i >= 0; i--){
      double nextEmax = 0;
      double nextEsum = 0;
      for(j = 0; j < g[i].size(); j++){
        nextEsum += E[g[i][j]];
        nextEmax = max(nextEmax, E[g[i][j]]);
      }
      if(i == x){
        E[i] = (nextEsum - nextEmax) / (g[i].size() - 1);
      }
      else{
        E[i] = nextEsum / g[i].size();
      }
      E[i] = E[i] + 1;
    }
    ans = min(ans, E[0]);
  }
  printf("%.10lf\n", ans);
  return 0;
}