#include <cstdio>
#include <vector>
#include <algorithm>
using namespace std;
int n,m,i,x,y,nr,crt,visited[100005],visitedt[100005];
vector <int> G[100005];
vector <int> GT[100005];
vector <int> sol;
vector <int> stiva;
void dfs(int node)
{
    visited[node]=1;
    for (int i=0; i<G[node].size(); i++)
    {
        int crt=G[node][i];
        if (!visited[crt]) dfs(crt);
    }
    stiva.push_back(node);
}
void dfst(int node)
{
    visitedt[node]=1;
    for (int i=0; i<GT[node].size(); i++)
    {
        int crt=GT[node][i];
        if (!visitedt[crt]) dfst(crt);
    }
    sol.push_back(node);
}
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",&x,&y);
        G[x].push_back(y);
        GT[y].push_back(x);
    }
    for (i=1; i<=n; i++) if (!visited[i]) dfs(i);
    nr=0;
    while (!stiva.empty())
    {
        crt=stiva.back();
        stiva.pop_back();
        if (!visitedt[crt])
        {
            dfst(crt);
            sol.push_back(-1);
            nr++;
        }
    }
    printf("%d\n",nr);
    reverse(sol.begin(),sol.end());
    while (!sol.empty())
    {
        crt=sol.back();
        sol.pop_back();
        if (crt==-1) printf("\n");
        else printf("%d ",crt);
    }
    return 0;
}