1 #include<bits/stdc++.h>
 2 #define N 100005
 3 #define LL long long
 4 #define inf 0x3f3f3f3f
 5 #define ls c[x][0]
 6 #define rs c[x][1]
 7 #define eps 1e-10  //这eps太扯淡了2333 
 8 using namespace std;
 9 inline int ra()
10 {
11     int x=0,f=1; char ch=getchar();
12     while (ch<‘0‘ || ch>‘9‘) {if (ch==‘-‘) f=-1; ch=getchar();}
13     while (ch>=‘0‘ && ch<=‘9‘) {x=x*10+ch-‘0‘; ch=getchar();}
14     return x*f;
15 }
16 int n,m,d[505];  
17 struct node{ int x,y; double z; }e[505*505]; double a[550][550];  
18 void guass()
19 {
20     for (int j=1; j<=n; j++)
21     {
22         bool flag=0; int pos;
23         for (int i=j; i<=n; i++) if (fabs(a[i][j])>eps) {pos=i; flag=1; break;}
24         if (!flag) continue; if (pos!=j) swap(a[j],a[pos]);
25         for (int i=j+1; i<=n; i++)
26             if (fabs(a[i][j])>eps)
27             {
28                 double t=a[i][j]/a[j][j];
29                 for (int k=j; k<=n+1; k++)
30                     a[i][k]-=t*a[j][k];
31             }
32     }
33     for (int i=n; i>=1; i--)
34     {
35         for (int j=i+1; j<=n; j++)
36             a[i][n+1]-=a[i][j]*a[j][n+1];
37         a[i][n+1]/=a[i][i];
38     }
39 }
40 bool cmp(node a, node b){return a.z>b.z;}
41 int main()
42 {
43     n=ra(); m=ra();
44     for (int i=1; i<=m; i++)
45         d[e[i].x=ra()]++,d[e[i].y=ra()]++;
46     for (int i=1; i<=m; i++)
47     {
48         a[e[i].x][e[i].y]+=1.0/d[e[i].y];
49         a[e[i].y][e[i].x]+=1.0/d[e[i].x];
50     }
51     for (int i=1; i<=n; i++) a[n][i]=0,a[i][i]=-1;
52     a[1][n+1]=-1; 
53     guass(); 
54     for (int i=1; i<=m; i++)
55         e[i].z=a[e[i].x][n+1]/d[e[i].x]+a[e[i].y][n+1]/d[e[i].y];
56     sort(e+1,e+m+1,cmp);
57     double ans=0;
58     for (int i=1; i<=m; i++)
59         ans+=i*e[i].z;
60     printf("%.3lf",ans);
61 }