#include<iostream.h>
#include<fstream.h>
#include<stdlib.h>
#define N 100001
#define M 200001
typedef struct nod
{long info;
struct nod *leg;}Nod,*list;
typedef struct stack
{long st[N],sp;};
stack s;
list q=NULL;
long n,m,a[M],b[M],deg[N]={0},*g[N],k,i,j,ind=0,c[N],l[N],nr,r;

void add(list &q,long x)
{Nod *p=new Nod;
p->info=x;
p->leg=q;
q=p;}

long del(list &q)
{Nod *p=q->leg;
long x=q->info;
free(q);
q=p;
return x;}

int contine(list &q,long x)
{Nod *p;
for(p=q;p!=NULL;p=p->leg)
if(p->info==x)
      return 1;
return 0;}

void push(stack &s,long x)
{s.st[++s.sp]=x;}

long pop(stack &s)
{return s.st[s.sp--];}

int apartine(stack &s,long x)
{long i;
for(i=1;i<=s.sp;i++)
if(s.st[i]==x)
      return 1;
return 0;}

void tarjan(long i,long *nr,long *ind)
{long j,r;
push(s,i);
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(apartine(s,g[i][j])==1&&l[i]>c[g[i][j]])
              l[i]=c[g[i][j]];
if(c[i]==l[i])
       {(*nr)++;
       while(s.sp!=0)
              {r=pop(s);
              if(contine(q,r)==0)
                   if(r!=i)
                         add(q,r);
                   else
                         break;
              else
                   break;}   
       if(contine(q,i)==0)
              add(q,i);
       r=del(q);                                
       if(r==0)
              (*nr)--;
       else
              add(q,r);
       add(q,0);}}

int main()
{ifstream f1("ctc.in");
ofstream f2("ctc.out");
f1>>n>>m;
for(k=1;k<=m;k++)
       {f1>>a[k]>>b[k];
       deg[a[k]]++;}
for(i=1;i<=n;i++)
       {g[i]=(long*)malloc(deg[i]*sizeof(long));
       deg[i]=0;
       c[i]=0;}
for(j=1;j<=m;j++)
       g[a[j]][deg[a[j]]++]=b[j];
ind=0;
nr=0;
s.sp=0;
q=NULL;
for(i=1;i<=n;i++)
if(c[i]==0)
       tarjan(i,&nr,&ind);
f2<<nr<<endl;
r=del(q);
while(q!=NULL)       
       {r=del(q);
       if(r!=0)
              f2<<r<<" ";
       else
              f2<<endl;}
f1.close();
f2.close();
return 0;}