#include <bits/stdc++.h>
#define nozerous(x) (x & -x)
#define MOD 9901
#define EPS 0.00001
using namespace std;
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int INF = (1 << 30), NMAX(100005), VMAX(1000000);
using VI  = vector<int>;
using VVI = vector<VI>;
using VB  = vector<bool>;
using Point = array<int, 2>;
using ll = long long;
using cd = complex<double>;
const double PI = acos(-1);
int n, s, v[NMAX], m;
bool viz[NMAX];
vector<int> G[NMAX];
vector<pair<int, int> > sol;
void DFS(int nod){
	viz[nod] = 1;
	for(auto it: G[nod])
		if(!viz[it]){
			sol.push_back({nod, it});
			DFS(it);
		}
}
int main()
{
    ifstream fin("mesaj4.in");
    ofstream fout("mesaj4.out");
    fin >> n >> m;
    for(int i = 1; i <= m; ++i){
		int x, y;
		fin >> x >> y;
		G[x].push_back(y);
		G[y].push_back(x);
	}
	DFS(1);
	for(int i = 1; i <= n; ++i)
		if(!viz[i]){
			fout << -1;
			return 0;
		}
	fout << 2 * n - 2 << '\n';
	for(int i = sol.size() - 1; i >= 0; --i)
		fout << sol[i].second << ' ' << sol[i].first << '\n';
	for(auto it: sol)
		fout << it.first << ' ' << it.second << '\n';
    return 0;
}