#include <cassert>
#include <fstream>
#include <algorithm>


using namespace std;

const int MAX_VERTEX = 100000, MAX_EDGE = 1000000;
int vertex;
int edge;
int start;
int stop;
int C[MAX_EDGE][2];

struct product {
  int pos;
  int flip;
} resu;

product searchedge(int arg[MAX_EDGE][2], int edge, int value){
    int valueLeft;
    int valueRight;
    product val;
    for (int i = 0; i < edge; i++){
        valueLeft=arg[i][0];
        valueRight=arg[i][1];
        if (valueLeft == value){
            val.pos = i;
            val.flip = 0;
            return val;
        }else if(valueRight == value){
            val.pos = i;
            val.flip = 1;
            return val;
        }
    }
}

int main() {
  ifstream fin("input.txt"); assert( fin );

  fin >> vertex >> edge >> start >> stop;

     for(int i = 0; i < edge; i++){
        fin >> C[i][0];
        fin >> C[i][1];
    }

       fin.close();

    int counter = edge;
    product position;
    ofstream fout("output.txt"); assert( fout );
    while (counter != 0){

        position = searchedge(C,edge,start);

        if (position.flip == 0){
                fout <<  C[position.pos][0]<< " " <<  C[position.pos][1] << endl;
                start = C[position.pos][1];
            }else{
                fout <<  C[position.pos][1]<< " " <<  C[position.pos][0] << endl;
                start = C[position.pos][0];
            }
        C[position.pos][0]=-1;
        C[position.pos][1]=-1;
        counter--;
    }
  fout.close();
  return 0;
}