#include<stdio.h>
#include<vector>
using namespace std;

vector <int> v[100006];

struct muc
{
    int x,y;
};
muc leg[200006];
int q[100006],sol,inc;
int n,m,viz[100006],sf;

int main ()
{
    int i,a,b,nr,nod;
    freopen("mesaj4.in","r",stdin);
    freopen("mesaj4.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;i++)
    {
        scanf("%d%d",&a,&b);
        v[a].push_back(b);
        v[b].push_back(a);
    }
    inc=sf=1;
    q[1]=1;
    viz[1]=1;
    while(inc<=sf)
    {
        nod=q[inc];
        nr=v[nod].size();
        for(i=0;i<nr;i++)
            if(!viz[v[nod][i]])
            {
                leg[++sol].x=nod;
                leg[sol].y=v[nod][i];
                viz[v[nod][i]]=1;
                q[++sf]=v[nod][i];
            }
        inc++;
    }
    for(i=1;i<=n;i++)
        if(!viz[i])
        {
            printf("-1\n");
            return 0;
        }
    printf("%d\n",2*sol);
    for(i=sol;i>=1;i--)
        printf("%d %d\n",leg[i].y,leg[i].x);
    for(i=1;i<=sol;i++)
        printf("%d %d\n",leg[i].x,leg[i].y);
    return 0;
}