#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <queue>

using namespace std;

ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");

const int DIM = 100005;

int n,m,x,y,viz[DIM];

vector <int> G[DIM];
vector < pair<int,int> > Sol;

void Read()
{
    fin>>n>>m;
    while(m--)
        {
            fin>>x>>y;
            G[x].push_back(y);
            G[y].push_back(x);
        }
}

void DFS(int nod)
{
    viz[nod]=1;
    vector <int> ::iterator it;
    for(it=G[nod].begin();it!=G[nod].end();it++)
        {
            if(!viz[*it])
                {
                    viz[*it]=1;
                    DFS(*it);
                    Sol.push_back(make_pair(*it,nod));
                }
        }
}

void Print()
{
   if(Sol.size()!=(n-1))
            fout<<-1<<'\n';
   else
        {
            fout<<(n-1)*2<<'\n';
            for(int i=0;i<Sol.size();i++)
                fout<<Sol[i].first<<" "<<Sol[i].second<<'\n';
            for(int i=Sol.size()-1;i>=0;i--)
                fout<<Sol[i].second<<" "<<Sol[i].first<<'\n';
        }
}

int main()
{
    Read();
    DFS(1);
    Print();
}