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 <iostream>
#include <vector>
#include <cstdio>

using namespace std;
typedef long long ll;
#define REP(i,n) for (int i = 0; i < (n); ++i)

int main(){
    int n, m;
    cin >> n >> m;
    vector<vector<int>> net(n);
    REP(i, m){
        int si, ti;
        cin >> si >> ti;
        si--; ti--;
        net.at(si).push_back(ti);
    }

    double ans = n + 1;
    for(int ng = -1; ng < n - 1; ++ng){
        if(ng != -1 && net.at(ng).size() == 1){
            continue;
        }
        vector<double> exp(n, 0);
        for(int x = n - 2; x >= 0; x--){
            double tmp = 0;
            double maxval = 0;
            for(int nxt : net.at(x)){
                tmp += exp.at(nxt) + 1.0;
                maxval = max(maxval, exp.at(nxt) + 1.0);
            }
            if(ng != x){
                exp.at(x) = tmp / (double)net.at(x).size();
            }else{
                exp.at(x) = (tmp - maxval) / (double)(net.at(x).size() - 1);
            }
        }
        ans = min(ans, exp.at(0));
    }
    printf("%.8f\n", ans);
    return 0;
}