#include <iostream>
#include <fstream>
#include <queue>

using namespace std;
ifstream in("graf.in");
ofstream out("graf.out");
const int NMAX = 7501;
int frcv[NMAX], distX[NMAX], distY[NMAX];
vector <int> v[NMAX];
int start, finish, n, nrmoduri;
void bfs( int start_node, int dist[]){
    queue <int> coada;
    coada.push(start_node);
    for( int i = 1 ; i <= n ; i++ )
        dist[i] = -1;
    dist[start_node] = 0;
    while(!coada.empty()){
        int nod = coada.front();
        coada.pop();
        for( int i = 0 ; i < v[nod].size(); i++ ){
          if( dist[v[nod][i]] == -1 ){
            coada.push(v[nod][i]);
            dist[v[nod][i]]=dist[nod]+1;
          }
        }
    }
}

int main()
{
    int m, a, b;
    in >> n >> m >> start >> finish;
    for( int i = 0 ; i < m ; i++ ){
        in >> a >> b;
        v[a].push_back(b);
        v[b].push_back(a);
    }
    bfs(start, distX);
    bfs(finish, distY);
    for( int i = 1 ; i <= n ; i++ )
        if( distX[i] + distY[i] == distX[finish] ){
            frcv[distX[i]]++;
        }
    vector <int> ans;
    for( int i = 1 ; i <= n ; i++ )
        if( distX[i] + distY[i] == distX[finish] && frcv[distX[i]] == 1 )
            ans.push_back(i);
    out << ans.size() << endl;
    for( int i = 0 ; i < ans.size() ; i++ )
        out << ans[i] << " ";
    return 0;
}