#include <iostream>
#include <fstream>
#include <vector>
#define NMAX 100005

using namespace std;

ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");

vector < int > g[NMAX];
vector < pair < int, int > > sol;

bool viz[NMAX];
int n, m;

void dfs(int nod){
	viz[nod] = 1;
	for(int i : g[nod]){
		if(!viz[i]){
			dfs(i);
			sol.push_back({i, nod});
		}
	}
}

int main(){
	fin >> n >> m;
	for(int i = 0, x, y; i < m; i++){
		fin >> x >> y;
		g[x].push_back(y);
		g[y].push_back(x);
	}
	fin.close();
	dfs(1);
	for(int i = 1; i <= n; i++)
		if(!viz[i]){
			fout << -1;
			return 0;
		}
	fout << 2 * sol.size() << '\n';
	for(auto i : sol)
		fout << i.first << ' ' << i.second << '\n';
	for(int i = sol.size() - 1; i >= 0; i--)
		fout << sol[i].second << ' ' << sol[i].first << '\n';
	fout.close();
	return 0;
}