#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <vector>

using namespace std;

#define file_in "mesaj4.in"
#define file_out "mesaj4.out"

#define nmax 101000

int n,m;
int nr;
int viz[nmax];
int a[nmax];
int b[nmax];
vector<int> G[nmax];
int grad[nmax];

void citire()
{
    int a,b;
    freopen(file_in,"r",stdin);
    freopen(file_out,"w",stdout);

    scanf("%d %d", &n, &m);
    while(m--)
    {
        scanf("%d %d ", &a, &b);

        G[a].push_back(b);
        G[b].push_back(a);
        grad[a]++;
        grad[b]++;
    }
}

void parc(int nod)
{
    vector<int> :: iterator it;
    //viz[nod]=1;

    for (it=G[nod].begin();it!=G[nod].end();++it)
          if (!viz[*it])
          {
              a[++nr]=nod;
              b[nr]=*it;
              viz[*it]=1;
              parc(*it);
          }
}


void solve()
{
    int i;
    for (i=1;i<=n;++i) if (grad[i]==0)
    {
        printf("-1\n");
        exit(0);;
    }
    nr=0;
    viz[1]=1;
    parc(1);
    printf("%d\n", 2*(n-1));
    for (i=nr;i>=1;--i)
         printf("%d %d\n", b[i], a[i]);
    for (i=1;i<=nr;++i)
         printf("%d %d\n",a[i], b[i]);
}

int main()
{
    citire();
    solve();

    fclose(stdin);
    fclose(stdout);

    return 0;
}