#include <vector>
#include <fstream>

using namespace std;

const int MaxN=100005;

const char InFile[]="mesaj4.in";
const char OutFile[]="mesaj4.out";

struct dr{int from,to;};

vector<dr> drum;
vector<int> a[MaxN];
ifstream fin(InFile);
ofstream fout(OutFile);
int N,M,x,y,viz[MaxN];

void dfs(int nod)
{
	if(!viz[nod])
	{
		viz[nod]=1;
		for(register int i=0;i<(int)a[nod].size();++i)
		{
			if(!viz[a[nod][i]])
			{
				dfs(a[nod][i]);
				dr d;
				d.from=a[nod][i];
				d.to=nod;
				drum.push_back(d);
			}
		}
	}
}

int main()
{
	fin>>N>>M;
	for(register int i=0;i<M;++i)
	{
		fin>>x>>y;
		a[x].push_back(y);
		a[y].push_back(x);
	}
	fin.close();
	
	dfs(1);
	
	if(drum.size()==N-1)
	{
		fout<<2*(N-1)<<"\n";
		for(register int i=0;i<(int)drum.size();++i)
		{
			fout<<drum[i].from<<" "<<drum[i].to<<"\n";
		}
		for(register int i=(int)drum.size()-1;i>=0;--i)
		{
			fout<<drum[i].to<<" "<<drum[i].from<<"\n";
		}
	}
	else
	{
		fout<<"-1";
	}
	fout.close();
	return 0;
}