#include <bits/stdc++.h>
using namespace std;
ifstream fi("in.in");
ofstream fo("out.out");

const int NMAX = 1e5 + 5;

int n, m;
vector <int> G[NMAX], H[NMAX];
bool viz[NMAX];
int stiva[NMAX], k;
int nrComp, comp[NMAX];
vector <int> bat[NMAX];

void dfs1(int nod)
{
    viz[nod] = 1;
    for (auto v: G[nod])
        if (!viz[v])
            dfs1(v);
    stiva[++k] = nod;
}

void dfs2(int nod)
{
    bat[nrComp].push_back(nod);
    comp[nod] = nrComp;

    for (auto v: H[nod])
        if (!comp[v])
            dfs2(v);
}

int main()
{
    fi >> n >> m;
    for (int i = 1; i <= m; i++)
    {
        int u, v;
        fi >> u >> v;
        G[u].push_back(v);
        H[v].push_back(u);
    }

    for (int i = 1; i <= n; i++)
        if (!viz[i])
            dfs1(i);

    for (int i = 1; i <= n; i++)
        if (!viz[i])
        {
            fo << 0;
            return 0;
        }

    while (k > 0)
    {
        int nod = stiva[k--];

        nrComp++;
        dfs2(nod);
    }

    int cntr = 0;
    vector <int> rez;
    for (int i = 1; i <= nrComp; i++)
    {
        bool rad = 1, fr = 1;
        for (auto nod: bat[i])
        {
            for (auto v: G[nod])
                if (comp[v] != comp[nod])
                    fr = 0;
            for (auto v: H[nod])
                if (comp[v] != comp[nod])
                    rad = 0;
        }
        if (rad)
            cntr++;
        if (fr)
            for (auto nod: bat[i])
                rez.push_back(nod);
    }

    if (cntr <= 1)
    {
        fo << rez.size() << "\n";
        for (auto x: rez)
            fo << x << " ";
    }

    return 0;
}