#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("ctc.in");
ofstream g("ctc.out");
#define nmax 100010
struct nod
{
    int val;
    nod *urm;
};
typedef nod *pnod;
pnod p,v[nmax];
void ad(int x,int y)
{
    p=new nod;
    p->val=y;
    p->urm=v[x]->urm;
    v[x]->urm=p;
}
int acum[2*nmax],ind[nmax],mind[nmax],st[nmax],nr,index,cate,ss,ind2;
bool instack[nmax];
void tarjan(int inc)
{
    index+=1;
    ss+=1;
    ind[inc]=mind[inc]=index;
    st[ss]=inc;
    instack[inc]=1;
    pnod p=v[inc]->urm;
    while(p)
    {
        if(ind[p->val]==0)
        {
            tarjan(p->val);
            mind[inc]=min(mind[inc],mind[p->val]);
        }
        else
            if(instack[p->val]==1)
                mind[inc]=min(mind[inc],mind[p->val]);
        p=p->urm;
    }
    if(ind[inc]==mind[inc])
    {
        cate+=1;
        ind2=nr+1;
        while(st[ss]!=inc)
        {
            nr+=1;
            acum[nr]=st[ss];
            instack[ss]=0;
            ss-=1;
        }
        instack[inc]=0;
        nr+=1;
        acum[nr]=inc;
        ss-=1;
        sort(acum+ind2,acum+nr+1);
        nr+=1;
    }
}
int main()
{
    int n,m,i,x,y;
    f>>n>>m;
    for(i=1;i<=n;i++)
    {
        v[i]=new nod;
        v[i]->urm=0;
    }
    for(i=1;i<=m;i++)
    {
        f>>x>>y;
        ad(x,y);
    }
    for(i=1;i<=n;i++)
        if(ind[i]==0)
        {
            index=0;
            tarjan(i);
        }
    g<<cate<<'\n';
    for(i=1;i<=nr;i++)
    {
        if(acum[i]==0)
            g<<'\n';
        else
            g<<acum[i]<<" ";
    }
    return 0;
}