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