#include <iostream>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <algorithm>

using namespace std;

vector<vector<int> > g;
vector<int> order, used, check;
vector<pair<int, int> > edge;

void dfs(int v)
{
    used[v] = 1;
    for (int i = 0; i < g[v].size(); i++)
    {
        int to = g[v][i];
        if (!used[to])
            dfs(to);
    }
    order.push_back(v);
}

int main()
{
    int n, m;
    cin >> n >> m;

    g.resize(n);
    check.assign(n, 0);

    int x, y;
    for (int i = 0; i < n; i++)
    {
        cin >> x >> y;
        x--;
        y--;
        g[x].push_back(y);
    }

    used.assign(n, 0);
    for (int i = 0; i < n; i++)
    {
        if (!used[i])
            dfs(i);
    }

    check[0] = -1;

    for (int i = order.size() - 1; i >= 0; i--)
    {
        int v = order[i];
        for (int j = 0; j < g[v].size(); j++)
        {
            int to = g[v][j];
            edge.push_back({ v, to });
        }
    }

    for (int i = 0; i < edge.size(); i++)
    {
        int to = edge[i].second;
        int from = edge[i].first;
        if (check[from] == 0)
            continue;
        if (check[to] == -1)
            continue;
        if (check[to] == 0)
        {
            if (check[from] == -1)
                check[to] = i+1;
            else
            {
                check[to] = check[from];
            }
        }
        else
            if (check[from] == -1)
            {
                check[to] = -1;
            }
            else
            {
                if (check[from] != check[to])
                    check[to] = -1;
                /*if(check[to] > 0 && check[from] > 0)
                    check[to] = min(check[to], check[from]);*/
            }
    }

    int ans = 0;
    for (int i = 0; i < n; i++)
        if (check[i] <= 0)
            ans++;

    cout << ans << endl;

    for (int i = 0; i < n; i++)
        if (check[i] <= 0)
            cout << i + 1 << " ";

    return 0;
}