#include<fstream.h>
#define N 100001
long n,m,deg[N]={0},*g[N],k,i,j,t=0,ind,c[N],l[N],nr,a[2*N],b[2*N],u,p,r,v[N]={0},s[2*N],q[2*N],ss,sq;
void tarjan(long i,long *nr,long *ind)
{long j,r,k;
s[++ss]=i;
if(v[i]==0)
      v[i]=1;
c[i]=(*ind);
l[i]=(*ind)++;
for(j=0;j<deg[i];j++)
if(c[g[i][j]]==0)
      {tarjan(g[i][j],nr,ind);
      if(l[i]>l[g[i][j]])
             l[i]=l[g[i][j]];}
else
      if(v[g[i][j]]==1&&l[i]>c[g[i][j]])
             l[i]=c[g[i][j]];
if(c[i]==l[i])
      {(*nr)++;
      k=0;
      while(ss)
             {r=s[ss--];
             if(r!=i&&v[r]==1)
                     q[++sq]=r,k++,v[r]=2;
             else
                     break;}
      if(v[i]==1)
             v[i]=2,k++,q[++sq]=i;
      if(k==0)
             (*nr)--;
      else
             q[++sq]=0;}}
int main()
{ifstream x("ctc.in");
ofstream y("ctc.out");
x>>n>>m;
for(k=1;k<=m;k++)
      {x>>a[k]>>b[k];
      deg[a[k]]++;}
for(i=1;i<=n;deg[i++]=0)
      g[i]=(long*)malloc(deg[i]*sizeof(long));
for(j=1;j<=m;j++)
      g[a[j]][deg[a[j]]++]=b[j];
ind=nr=ss=sq=0;
for(u=1;u<=n;u++)
if(c[u]==0)
      tarjan(u,&nr,&ind);
y<<nr<<"\n";
r=q[sq--];
while(sq)
      {r=q[sq--];
      if(r==0)
              y<<"\n";
      else
              y<<r<<" ";}
x.close();
y.close();
return 0;}