#include <bits/stdc++.h>
#define Dim 100004
using namespace std;
ifstream f("mesaj4.in");
ofstream g("mesaj4.out");
vector < int > V[Dim];
queue < int > qu;
deque < pair<int,int> > tur,retur;
bool viz[Dim],ok=1;
int N,M,a,b;

void DFS()
{
    qu.push(1);
    viz[1]=1;
    while(!qu.empty())
    {
        int nod=qu.front();
        qu.pop();
        for(unsigned int i=0;i<V[nod].size();i++)
        {
            int vecin=V[nod][i];
            if(!viz[vecin])
            {
                viz[vecin]=1;
                qu.push(vecin);
                tur.push_back(make_pair(nod,vecin));
                retur.push_front(make_pair(vecin,nod));
            }
        }
    }
}

int main()
{
    f>>N>>M;
    for(int i=1;i<=M;i++)
    {
        f>>a>>b;
        V[a].push_back(b);
        V[b].push_back(a);
    }
    DFS();
    for(int i=1;i<=N;i++) if(viz[i]==0) ok=0;
    if(ok)
   {
       g<<(2*(N-1))<<'\n';
    while(!retur.empty())
    {
        g<<retur.front().first<<" "<<retur.front().second<<'\n';
        retur.pop_front();
    }
    while(!tur.empty())
    {
        g<<tur.front().first<<" "<<tur.front().second<<'\n';
        tur.pop_front();
    }

   }else g<<-1;
    return 0;
}