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#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <vector>
#include <cmath>
#include <cstring>
#include <string>
#include <iterator>
#include <iomanip>
#include <queue>
#include <stack>
using namespace std;
 
#define sqr(x) ((x)*(x))
#define cbr(x) ((x)*(x)*(x))
#define rep(c, i, n) for((i)=(c); (i)<(n); (i)++)
/**
//Geometry
 
struct pt { long long x, y; };
double vecmul(pt a, pt b) { return (a.x*b.y - b.x*a.y); }
 
double dist(double ax, double ay, double bx, double by) { return sqrt(sqr(ax - bx) + sqr(ay - by)); }
*/
 
//DBG
template<typename T, typename T2>void printarr(T  a[], T2 sz, T2 beg = 0) { for (T2 i = beg; i<sz; i++) cout << a[i] << " "; cout << endl; }
#define DBG(a)         cout<<#a<<"="<<(a)<<"\n"
#define DBG2(a,b)       cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<"\n"
#define DBG3(a,b,c)     cout<<#a<<"="<<(a)<<", "<<#b<<"="<<(b)<<", "<<#c<<"="<<(c)<<"\n"
 
//Files
#define FILE_MODE(FILE) freopen(FILE".in", "r", stdin), freopen(FILE".out", "w", stdout)
 
//Constants
#define PI    3.1415926535897932
#define INF   1011111111
#define LLINF 1000111000111000111LL
#define eps   1e-14
#define mod   1000000007
#define ll long long
#define ull unsigned long long
//-------------------------------------------------
#define N 50010
//7 9 1 7 3 6 1 3 6 7 1 4 5 6  1 2 2 5 3 5  4 7 
int n, m, s, f;
vector<int> vertex[N];
queue<int> bfsQ;
vector<int> path[N];
int vis[N];
int dist[N];
 
int main() {
    cin >> n >> m >> s >> f;
    s--; f--;
    int i,x,y,j;
    rep(0,i,m) {
        cin >> x >> y;
        x--; y--;
        vertex[x].push_back(y);
        vertex[y].push_back(x);
    }
    //visited 0, distance to everything is inf.
    memset(dist, -1, sizeof dist);
    memset(vis, 0, sizeof vis);
 
    dist[s] = 0;
    vis[s] = 1;
    path[s].push_back(s);
 
    bfsQ.push(s);
    int minDist=-1;
    while (!bfsQ.empty()) {
        int u = bfsQ.front();
 
        bfsQ.pop();
 
        for (int i : vertex[u]) {
            if (!vis[i]) {
                vis[i] = 1;
                dist[i] = dist[u]+1;
                path[i] = path[u];
                path[i].push_back(i);
                bfsQ.push(i);
            }
        }
    }
    cout << dist[f] << endl;
    rep(0, i, path[f].size()) {
        path[f][i]++;
    }
    copy(path[f].begin(), path[f].end(), ostream_iterator<int>(cout, " "));
    return 0;
}