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;
const double inf = 1e100;

int n, m;
bool G[600][600];
int C[600];
double P[600];
double E[600];
int main() {
  scanf("%d %d", &n, &m);
  for (int j = 0; j < m; ++j) {
    int a, b;
    scanf("%d %d", &a, &b);
    --a; --b;
    G[a][b] = true;
    ++C[a];
  }
  
  P[0] = 1;
  for (int i = 0; i < n; ++i) {
    for (int j = i + 1; j < n; ++j) {
      if (!G[i][j]) continue;
      P[j] += P[i] / C[i];
    }
  }
  
  for (int i = n - 2; i >= 0; --i) {
    double sum = 0;
    for (int j = i + 1; j < n; ++j) {
      if (!G[i][j]) continue;
      sum += E[j];
    }
    E[i] = sum / C[i] + 1;
  }
  
  double mn = 0;
  for (int i = 0; i < n; ++i) {
    if (C[i] <= 1) continue;
    for (int j = i + 1; j < n; ++j) {
      if (!G[i][j]) continue;
      double ans = P[i] * (E[i] - E[j] - 1) / (C[i] - 1);
      mn = min(mn, ans);
    }
  }
  printf("%.16f", E[0] + mn);
}