1 #include<iostream>
  2 #include<cstring>
  3 #include<cstdio>
  4 using namespace std;
  5 inline int read(){
  6     int sum(0);
  7     char ch(getchar());
  8     for(;ch<‘0‘||ch>‘9‘;ch=getchar());
  9     for(;ch>=‘0‘&&ch<=‘9‘;sum=sum*10+(ch^48),ch=getchar());
 10     return sum;
 11 }
 12 struct edge{
 13     int s,e,n;
 14 }a[300001],b[100001];
 15 int pre[100001],tot;
 16 int adj[100001],ttt;
 17 inline void insert(int s,int e){
 18     a[++tot].s=s;
 19     a[tot].e=e;
 20     a[tot].n=pre[s];
 21     pre[s]=tot;
 22 }
 23 inline void add(int s,int e){
 24     b[++ttt].s=s;
 25     b[ttt].e=e;
 26     b[ttt].n=adj[s];
 27     adj[s]=ttt;
 28 }
 29 int n,m;
 30 int stack[100001],top;
 31 int cnt,low[100001],dfn[100001],bl[100001],qlt;
 32 bool vis[100001];
 33 int size[100001];
 34 inline int my_min(int a,int b){
 35     return a<b?a:b;
 36 }
 37 inline void tarjan(int u){
 38     cnt++;
 39     vis[u]=1;
 40     low[u]=dfn[u]=cnt;
 41     stack[++top]=u;
 42     for(int i=pre[u];i!=-1;i=a[i].n){
 43         int e(a[i].e);
 44         if(!dfn[e]){
 45             tarjan(e);
 46             low[u]=my_min(low[u],low[e]);
 47         }
 48         else
 49             if(vis[e])
 50                 low[u]=my_min(low[u],dfn[e]);
 51     }
 52     if(low[u]==dfn[u]){
 53         int tmp;
 54         qlt++;
 55         while(1){
 56             tmp=stack[top--];
 57             bl[tmp]=qlt;
 58             vis[tmp]=0;
 59             if(tmp==u)
 60                 break;
 61         }
 62     }
 63 }
 64 bool flag[100001];
 65 inline void uni(int u){//坑点处理dfs
 66     for(int i=adj[u];i!=-1;i=b[i].n){
 67         int e(b[i].e);
 68         if(!vis[e]){
 69             vis[e]=1;
 70             uni(e);
 71             size[u]+=size[e];
 72         }
 73     }
 74 }
 75 int ans(0);
 76 int ind[100001];
 77 int main(){
 78 //  freopen("killer.in","r",stdin);
 79 //  freopen("killer.out","w",stdout);
 80     memset(pre,-1,sizeof(pre));
 81     memset(adj,-1,sizeof(adj));
 82     n=read(),m=read();
 83     for(int i=1;i<=m;i++){
 84         int x(read()),y(read());
 85         insert(x,y);
 86     }
 87     for(int i=1;i<=n;i++)
 88         if(!dfn[i])
 89             tarjan(i);
 90     for(int i=1;i<=n;i++)
 91         size[bl[i]]++;
 92     for(int i=1;i<=m;i++){
 93         int s(a[i].s),e(a[i].e);
 94         if(bl[s]!=bl[e])
 95             add(bl[s],bl[e]),ind[bl[e]]++;
 96     }
 97     for(int i=1;i<=qlt;i++)
 98         if(!ind[i]){//坑点特判
 99             uni(i);
100             if(size[i]==1){
101                 ans=-1;
102                 break;
103             }
104         }
105     for(int i=1;i<=qlt;i++)
106         if(ind[i]==0)
107             ans++;
108     printf("%.6lf",(double)(n-ans)/(double)n);
109 }