#include <vector>
#include <iostream>
#include <algorithm>

using namespace std;

vector<int> in[100000], out[100000];
int used[100000];
vector<int> done;
int prev1[100000];
vector<int> ans, res;
int a, b;

int dfs(int u)
{
    if (used[u] == 1)
        return 0;
    if (u == b)
    {
        int v = b;
        while (v != -1)
        {
            res.push_back(v);
            v = prev1[v];
        }
        reverse(res.begin(), res.end());

        return 1;
    }

    used[u] = 1;
    for (int j = 0; j < out[u].size(); j++)
    {
        int v = out[u][j];
        if (!used[v])
        {
            prev1[v] = u;
            if (dfs(v))
                return 1;
        }
    }
    return 0;
}

int main()
{
    cout.sync_with_stdio(0);
    int n, m, p, q;
    cin >> n >> m >> a >> b;
    a--, b--;

    for (int i = 0; i < n; i++)
        used[i] = 0;

    for (int i = 0; i < m; i++)
    {
        cin >> p >> q;
        out[p - 1].push_back(q - 1);
        in[q - 1].push_back(p - 1);
    }

    prev1[a] = -1;

    dfs(a);

    int cnt = 0;
    int k = (int)res.size();
    int up[k], down[k];
    for (int i = 0; i < k; i++)
    {
        up[i] = 0;
        if (!i)
            down[i] = 0;
        else
            down[i] = 1;
    }
    for (int i = 0; i < (int)res.size(); i++)
    {
        if (i > 0)
            up[i - 1] = -1;
        down[i] = 0;
        for (int j = 0; j < in[i].size(); j++)
        {
            int v = in[i][j];
            cnt += up[v];
        }
        //cout << res[i] + 1 << " " << cnt << endl;
        if (cnt == 0 && res[i] != a && res[i] != b)
            ans.push_back(res[i] + 1);
        for (int j = 0; j < out[i].size(); j++)
        {
            int v = out[i][j];
            cnt += down[v];
        }
    }
    cout << ans.size() << endl;
    sort(ans.begin(), ans.end());
    for (int i = 0; i < (int)ans.size(); i++)
        cout << ans[i] << " ";
}