#include <algorithm>
#include <vector>
#include <bitset>
using namespace std;

#define pb push_back
#define mp make_pair
#define DIM 100005
#define sc second
#define fs first

vector <pair <int,int> > sol;
vector <int> g[DIM];
bitset <DIM> viz;
int n,m;

void read ()
{
    int i,x,y;

    scanf ("%d%d",&n,&m);
    for (i=1; i<=m; ++i)
    {
        scanf ("%d%d",&x,&y);
        g[x].pb (y);
        g[y].pb (x);
    }
}

void df (int nod)
{
    vector <int> :: iterator it;

    viz[nod]=1;
    for (it=g[nod].begin (); it!=g[nod].end (); ++it)
        if (!viz[*it])
        {
            sol.pb (mp (nod,*it));
            df (*it);
        }
}

void print ()
{
    vector <pair <int,int> > :: reverse_iterator it1;
    vector <pair <int,int> > :: iterator it2;

    if ((int)sol.size ()!=n-1)
        printf ("-1");
    else
    {
        printf ("%d\n",2*n-2);
        for (it1=sol.rbegin (); it1!=sol.rend (); ++it1)
            printf ("%d %d\n",it1->sc,it1->fs);
        for (it2=sol.begin (); it2!=sol.end (); ++it2)
            printf ("%d %d\n",it2->fs,it2->sc);
    }
}

int main ()
{
    freopen ("mesaj4.in","r",stdin);
    freopen ("mesaj4.out","w",stdout);

    read ();
    df (1);
    print ();

    return 0;
}