#include<fstream>
#include<vector>
#define max 100001
using namespace std;

int a[max],b[max],viz[max],c[max];
vector <int>lad[max];
vector <int>::iterator it;
int main()
{
	int n,m,i,p,u,z,x,y;
	ifstream f("mesaj4.in");
	ofstream g("mesaj4.out");
	f>>n>>m;
	for(i=1;i<=m;i++)
		{f>>x>>y;
	     lad[x].push_back(y);
		 lad[y].push_back(x);
		}
	//BF(1)
	int nr=1,j=0;
	viz[1]=1;c[1]=1;
	u=p=1;
	while(p<=u)
		{z=c[p];p++;
	     for(it=lad[z].begin();it!=lad[z].end();++it)
			if(viz[*it]==0)
			   {++u;
			    c[u]=*it;
				viz[*it]=1;
				nr++;
				
				j++;a[j]=z;b[j]=*it;
			   }
		}
	if(nr!=n)g<<-1<<'\n';
	else
    	{g<<2*n-2<<'\n';
		for(i=n-1;i>=1;i--)g<<b[i]<<' '<<a[i]<<'\n';	
		  for(i=1;i<n;i++)g<<a[i]<<' '<<b[i]<<'\n';
		}
	}