#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);
	}
	dfs(1);
	int size = sol.size();
	if(size < n-1){
		fout << -1;
		return 0;
	}
	fout << 2 * size << '\n';
	for(int i = 0; i < size; i++)
		fout << sol[i].first << ' ' << sol[i].second << '\n';
	for(int i = size - 1; i >= 0; i--)
		fout << sol[i].second << ' ' << sol[i].first << '\n';
	return 0;
}