#include <bits/stdc++.h>

using namespace std;

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

int n, m, x, y; bool viz[100100];
vector <int> v[100100]; vector <pair <int, int> > sol;

void dfs(int x){
	viz[x] = 1;
	for(int k = 0; k < v[x].size(); k++)
		if(!viz[v[x][k]]){
			sol.push_back(make_pair(v[x][k], x));
			dfs(v[x][k]);
		}
}

int main(){
	in >> n >> m;
	for(int i = 1; i <= m; i++){
		in >> x >> y;
		v[x].push_back(y);
		v[y].push_back(x);
	}
	dfs(1);
	for(int i = 1; i <= n; i++) if(!viz[i]) return out << -1, 0;
	out << 2*n-2 << '\n';
	for(int i = n-2; i >= 0; i--){
		out << sol[i].first << ' ' << sol[i].second << '\n';
	}
	for(int i = 0; i < n-1; i++){
		out << sol[i].second << ' ' << sol[i].first << '\n';
	}
	return 0;
}