#include <stdio.h>
#include<cstring>
#include<fstream>
#include<vector>
using namespace std;
typedef struct{
  int g,h;
}element;
element da[200002];
ofstream fo("mesaj4.out");
int ok=0;
int nr=0,vizitate=0,n, m,mm, viz[100005];
typedef struct{
int n1,n2;
} afisare;
afisare afis[100005];
vector<int>v[100005];

void citire()
{
	freopen("mesaj4.in","r",stdin);
	scanf("%d %d",&n,&m);
	int i, x, y;

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

}

void DFS(int nod,int precedent)
{
  int i,elem;
  elem=v[nod].size();
	viz[nod] = 1;
	vizitate++;
	if(precedent!=0) {afis[++mm].n1=precedent; afis[mm].n2=nod; }
	for (i=0; i<=elem-1; i++) if (!viz[v[nod][i]]) DFS(v[nod][i],nod);
    if(precedent!=0) {
      da[++nr].g=nod;
      da[nr].h=precedent;
    }

}

int main()
{
  int i;
	memset(viz,0,sizeof(viz));
	citire();
	DFS(1,0);
  if(vizitate!=n) {fo<<"-1\n"; return 0;
  }
  fo<<2*n-2<<"\n";
  for(i=1;i<=nr;i++)
  fo<<da[i].g<<" "<<da[i].h<<"\n";
	for(i=1;i<=mm;i++)
        fo<<afis[i].n1<<" "<<afis[i].n2<<"\n";
  fo.close();
	return 0;
}