#include <fstream>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;
ifstream in ("graf.in");
ofstream out ("graf.out");
vector <int> v[14002];
queue <int> c;
int n,m,k,x,y,i,p,d[14002];
int sol[14002],nr1;
struct kys {int a,b;};
kys viz[14002];
int comp (kys x, kys y)
{
    return x.a<y.a;
}
void bfs(int k)
{
    int i;
    c.push(k);
    viz[k].a=1;
    viz[k].b=k;
    d[viz[k].a]++;
    while(!c.empty())
    {
        int nod=c.front();
        c.pop();
        for(i=0;i<v[nod].size();i++)
        {
            if(!viz[v[nod][i]].a)
            {
                c.push(v[nod][i]);
                viz[v[nod][i]].a=viz[nod].a+1;
                viz[v[nod][i]].b=v[nod][i];
                d[viz[v[nod][i]].a]++;
            }
        }
    }
}
int main()
{
in>>n>>m>>k>>p;
for(i=1;i<=m;i++)
{
    in>>x>>y;
    v[x].push_back(y);
    v[y].push_back(x);

}
bfs(k);
sort(viz+1,viz+n+1,comp);
for(i=1;i<=n;i++)
{
    if(viz[i].b==p){sol[++nr1]=p;break;}
    if(d[viz[i].a]==1)sol[++nr1]=viz[i].b;
}
sort(sol+1,sol+nr1+1);
out<<nr1<<'\n';
for(i=1;i<=nr1;i++)
{
    out<<sol[i]<<" ";
}
    return 0;
}