#include <iostream>
#include <fstream>
#include <vector>
#define NMax 100005
using namespace std;
vector <int> Graf[NMax];
vector <pair< int,int > > Sol;
bool Uz[NMax];
int N,M,x,y,it;

void Citire()
{
    ifstream g("mesaj4.in");
    g>>N>>M;
    for(int i=1;i<=M;i++)
    {
        g>>x>>y;
        Graf[x].push_back(y);
        Graf[y].push_back(x);
    }
}

void DFS(int nod)
{
    Uz[nod]=1;
    for(int i=0;i<Graf[nod].size();i++)
    {
        int it=Graf[nod][i];
        if(Uz[it]==0)
        {
            DFS(it);
            Sol.push_back(make_pair(it,nod));
        }
    }
}

int main()
{
    Citire();
    DFS(1);
    ofstream f("mesaj4.out");
    if(Sol.size()!=N-1)
    {
         f<<-1<<"\n";
         return 0;
    }

    f<<2*Sol.size()<<"\n";

    for(int i=0;i<Sol.size();i++)
        f<<Sol[i].first<<' '<<Sol[i].second<<"\n";

    for(int i=Sol.size()-1;i>=0;i--)
        f<<Sol[i].second<<' '<<Sol[i].first<<"\n";
}