#include<stdio.h>
#include<vector>
#define LMAX 100000 + 1
using namespace std;
vector<int> pr[LMAX]; //contine toti prietenii lui i folosind containeru vector
int f[LMAX],a[LMAX],b[LMAX];
long elf;
void dfs(int nod)
{
long i;
f[nod]=1;
for(i=0;i<pr[nod].size();i++)
   if(f[pr[nod][i]]==0)
     {
     elf++; a[elf]=nod; b[elf]=pr[nod][i];
     dfs(pr[nod][i]);
     }
}
int main()
{
long n,m,i,val1,val2;
freopen("mesaj4.in","r",stdin);
freopen("mesaj4.out","w",stdout);
scanf("%ld%ld",&n,&m);
for(i=1;i<=m;i++)
  {
  scanf("%ld%ld",&val1,&val2);
  pr[val1].push_back(val2);   //val1 e prieten cu val2
  pr[val2].push_back(val1);   //val2 e prieten cu val1
  }
dfs(1);
if(elf+1!=n) //grafu nu e conex
  {printf("-1"); return 0; }
printf("%ld\n",n+n-2);
for(i=n-1;i>=1;i--)
  printf("%d %d\n",b[i],a[i]);
for(i=1;i<n;i++)
  printf("%d %d\n",a[i],b[i]);
return 0;
}