HARDDAG

#include <iostream>
#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <stack>
#include <queue>
#define task ""
using namespace std;
using ll = long long;
using ld = long double;

const int N = 15e4 + 2;
const int Inf = 1e9 + 7;
int n, m, l(0), low[N], num[N], cnt(0);
int par[N], x[N], deg[N], pos[N], in[N], out[N];
bool ok[N];
vector<int> adj[N],
    direct[N],
    lt[N],
    st,
    nadj[N];

int findpar(int v)
{
    return par[v] < 0 ? v : par[v] = findpar(par[v]);
}

void Read()
{
    cin >> n >> m;
    memset(par, -1, sizeof par);
    for (int i = 1; i <= m; ++i)
    {
        int u, v;
        cin >> u >> v;
        int x = findpar(u),
            y = findpar(v);
        if (x != y)
        {
            par[x] += par[y];
            par[y] = x;
        }
        direct[u].push_back(v);
    }
}

void dfs(int v)
{
    num[v] = low[v] = ++cnt;
    st.push_back(v);
    for (auto i : direct[v])
        if (num[i])
            low[v] = min(low[v], num[i]);
        else
        {
            dfs(i);
            low[v] = min(low[v], low[i]);
        }
    if (low[v] == num[v])
    {
        ++l;
        int t;
        do
        {
            t = st.back();
            lt[l].push_back(t);
            st.pop_back();
            low[t] = num[t] = Inf;
            par[t] = l;
        } while (v != t);
    }
}

void tarjan()
{
    for (int i = 1; i <= n; ++i)
        if (!num[i])
            dfs(i);
}

void topoSort()
{
    stack<int> q;
    for (int i = 1; i <= l; ++i)
    {
        deg[i] = nadj[i].size();
        if (deg[i] == 0)
            q.push(i);
    }
    int piv = 0;
    while (q.size())
    {
        x[++piv] = q.top();
        pos[x[piv]] = piv;
        q.pop();
        for (auto i : adj[x[piv]])
        {
            --deg[i];
            if (deg[i] == 0)
                q.push(i);
        }
    }
}

void Solve()
{
    vector<bool> ck(N);
    int check = 0;
    for (int i = 1; i <= n; ++i)
    {
        int v = findpar(i);
        if (!ck[v])
        {
            ++check;
            ck[v] = 1;
        }
    }
    if (check > 1)
    {
        cout << 0;
        return;
    }
    tarjan();

    for (int i = 1; i <= n; ++i)
    {
        for (auto j : direct[i])
        {
            if (par[i] != par[j])
            {
                adj[par[i]].push_back(par[j]);
                nadj[par[j]].push_back(par[i]);
            }
        }
    }
    for (int i = 1; i <= l; ++i)
    {
        sort(adj[i].begin(), adj[i].end());
        adj[i].resize(unique(adj[i].begin(), adj[i].end()) - adj[i].begin());
        sort(nadj[i].begin(), nadj[i].end());
        nadj[i].resize(unique(nadj[i].begin(), nadj[i].end()) - nadj[i].begin());
        in[i] = nadj[i].size();
        out[i] = adj[i].size();
    }
    topoSort();
    vector<int> a;
    int Lef(0), Rig(0);
    for (int i = 1; i <= l; ++i)
        if (in[x[i]] == 0)
            ++Rig;
    for (int i = 1; i <= l; ++i)
    {
        if (in[x[i]] == 0)
            --Rig;
        for (auto j : nadj[x[i]])
        {
            if (out[j] == 0)
                --Lef;
            ++out[j];
        }
        if (Lef == 0 && Rig == 0)
        {
            for (auto j : lt[x[i]])
                a.push_back(j);
        }
        for (auto j : adj[x[i]])
        {
            --in[j];
            --out[x[i]];
            if (in[j] == 0)
                ++Rig;
        }
        if (out[x[i]] == 0)
            ++Lef;
    }
    sort(a.begin(), a.end());
    cout << a.size() << "\n";
    for (auto i : a)
        cout << i << " ";
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    if (fopen(task ".INP", "r"))
        freopen(task ".INP", "r", stdin),
            freopen(task ".OUT", "w", stdout);
    Read();
    Solve();
}
/*
5 5
1 2
2 3
3 5
4 5
2 4
*/