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 int mx = 6e2;
double f[mx], g[mx];
vector<int> adj[mx];
int n, m;

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    cin >> n >> m;
    for(int i = 0; i < m; i++){
        int x, y;
        cin >> x >> y;
        x--, y--;
        adj[x].push_back(y);
    }
    for(int i = n - 2; i >= 0; i--){
        for(int v : adj[i]) f[i] += f[v] + 1;
        f[i] /= (adj[i].size());
    }

    double ans = f[0];
    for(int i = 0; i < n; i++){
        if(adj[i].size() <= 1) continue;
        double mxn = 0;
        for(int v : adj[i]) mxn = max(mxn, f[v] + 1);
        copy(f, f + n, g);
        g[i] = (g[i] * adj[i].size() - mxn) / (adj[i].size() - 1);
        for(int j = i - 1; j >= 0; j--){
            g[j] = 0;
            for(int v : adj[j]) g[j] += g[v] + 1;
            g[j] /= adj[j].size();
        }
        ans = min(ans, g[0]);
    }

    cout << fixed << setprecision(10) << ans << endl;
}