#include<fstream>
#include<vector>
using namespace std;
ifstream F("ctc.in");
ofstream G("ctc.out");
#define N 100001
vector<int> g[N];
int n,m,i,d,c[N],l[N],r,v[N],s[2*N],q[2*N],u,t;
void T(int i)
{
	int k,j,o,w;
	s[++t]=i,v[i]=!v[i]?1:v[i],l[i]=c[i]=d++;
	for(w=g[i].size(),j=0;j<w;++j)
        if(!c[g[i][j]]) {
            T(g[i][j]);
            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]) {
      	for(k=0,++r;t;) {
            o=s[t--];
            if(o==i||v[o]!=1)
                break;
            q[++u]=o,++k,v[o]=2;
        }
      	if(v[i]==1)
            v[i]=2,++k,q[++u]=i;
      	if(!k)
            --r;
      	else
            q[++u]=0;
	}
}
int main()
{
	F>>n>>m;
	while(m--)
        F>>i>>d,g[i].push_back(d);
	for(i=1;i<=n;++i)
        if(!c[i])
            T(i);
    for(G<<r<<"\n",--u;u;--u)
        if(!q[u])
            G<<"\n";
        else
            G<<q[u]<<" ";
    return 0;
}