#include<stdio.h>
#include<stdlib.h>
#define N 100001
#define M 200001
typedef struct stack
{long st[N],sp;};
stack s;
long n,m,a[M],b[M],deg[N]={0},*g[N],k,i,j,t,ind,c[N]={0},l[N],nr,d[N]={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 dfs(long i)
{long j;
d[i]=1;
for(j=deg[i]-1;j>=0;j--)
if(d[g[i][j]]==0)
       dfs(g[i][j]);}
              
void tarjan(long i)
{long j,r;
c[i]=ind;
l[i]=ind;
ind++;
if(apartine(s,i)==0)
       push(s,i);
for(j=deg[i]-1;j>=0;j--)
if(c[g[i][j]]==0)
       {tarjan(g[i][j]);
       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])
       {while(s.sp!=0)
              printf("%ld ",pop(s));}}

int main()
{freopen("ctc.in","r",stdin);
freopen("ctc.out","w",stdout);
scanf("%ld%ld",&n,&m);
for(k=1;k<=m;k++)
       {scanf("%ld%ld",&a[k],&b[k]);
       deg[b[k]]++;}
for(i=1;i<=n;i++)
       {g[i]=(long*)malloc((deg[i]+1)*sizeof(long));
       deg[i]=0;}
for(j=1;j<=m;j++)
       g[b[j]][deg[b[j]]++]=a[j];
nr=0;
for(i=1;i<=n;i++)
if(d[i]==0)
       {nr++;
       dfs(i);}
printf("%ld\n",nr);
for(k=1;k<=n;k++)
if(c[k]==0)
       {s.sp=0;
       tarjan(k);}
fclose(stdin);
fclose(stdout);
return 0;}