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;
using ld=long double;
typedef pair<int,int> P;
ld dp[605],EPS=1e-15,ans=1e9;
template<typename T>
void chmin(T &a,T b){
  if(a>b)a=b;
}

signed main(){
  int n,m;cin>>n>>m;
  vector<int> edge[n];
  while(m--){
    int s,t;cin>>s>>t;
    edge[--s].push_back(--t);
  }
  ld ans=1e9;
  for(int i=0;i<=n;i++){
    if(edge[i].size()==1)continue;
    memset(dp,0,sizeof(dp));
    for(int j=n-1;j>=0;j--){
      if(i==j){
        ld tmp=0;int a;
        for(int p:edge[j])if(tmp<dp[p])tmp=dp[p],a=p;
        for(int p:edge[j])if(p!=a)dp[j]+=(dp[p]+1)/((ld)edge[j].size()-1);
      }
      else for(int p:edge[j])dp[j]+=(dp[p]+1)/(ld)edge[j].size();
    }
    if(dp[0]>EPS)chmin(ans,dp[0]);
  }
  cout<<fixed<<setprecision(12)<<ans<<endl;
}