#include <iostream>
#include <fstream>
#include <bitset>
#include <queue>

#define pii pair<int, int>

using namespace std;

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

const int N = 1e5 + 1;
vector <int> adj[N];
bitset <N> viz;
pii rasp[N];
int index = 0;

void dfs(int nod) {
	viz[nod] = 1;
	for (auto vec : adj[nod]) {
		if (!viz[vec]) {
			rasp[index] = { vec, nod }, index++;
			dfs(vec);
		}
	}
}

int main() {
	int n, m;
	fin >> n >> m;
	for (int i = 1; i <= m; i++) {
		int a, b;
		fin >> a >> b;
		adj[a].push_back(b);
		adj[b].push_back(a);
	}
	dfs(1);
	if (index == n - 1) {
		fout << index * 2 << "\n";
		for (int i = 0; i < index; i++)
			fout << rasp[index - i - 1].first << " " << rasp[index - i - 1].second << "\n";
		for (int i = 0; i < index; i++)
			fout << rasp[i].second << " " << rasp[i].first << "\n";
	} else fout << -1;
	return 0;
}