#include<stdio.h>
#include<stdlib.h>
int n,m,w[100001],*g[100001],i,d,c[100001],l[100001],r,a[200002],b[200002],v[100001],s[200002],q[200002],t,u;
void T(int i) {
int k,j;
s[++t]=i,v[i]=!v[i]?1:v[i],l[i]=c[i]=d++;
for(j=0;j<w[i];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;q[++u]=s[t],k++,v[s[t--]]=2)
if(s[t]==i||v[s[t]]!=1)
break;
if(v[i]==1)
v[i]=2,k++,q[++u]=i;
if(!k)
r--;
else
q[++u]=0;
}
}
int main() {
freopen("ctc.in","r",stdin),freopen("ctc.out","w",stdout),scanf("%d%d",&n,&m);
for(i=1;i<=m;i++)
scanf("%d%d",&a[i],&b[i]),w[a[i]]++;
for(i=1;i<=n;w[i++]=0)
g[i]=(int*)malloc(w[i]*sizeof(int));
for(i=1;i<=m;i++)
g[a[i]][w[a[i]]++]=b[i];
for(i=1;i<=n;i++)
if(!c[i])
T(i);
printf("%d\n",r);
for(u--;u;u--)
if(!q[u])
printf("\n");
else
printf("%d ",q[u]);
}