#include <stdio.h>
#include <vector>
#include <algorithm>
#include <math.h>
#include <string.h>
 
using namespace std;

#define MAXN 100005

int n, m;

vector<int> g[MAXN];
char v[MAXN];
vector<pair<int, int>> sol;

void dfsCon(int x) {
	if (v[x]) {
		return;
	}
	v[x] = 1;
	for (auto y : g[x]) {
		dfsCon(y);
	}
}

bool iSConnected() {
	dfsCon(1);
	for (int i = 1; i <= n; ++i) {
		if (!v[i]) {
			return false;
		}
	}

	return true;
}

void dfsAquire(int x, int p) {
	bool canVisit = false;
	v[x] = 2;
	for (auto y : g[x]) {
		if (v[y] < 2) {
			dfsAquire(y, x);
		}
	}
	if (p != 0) {
		sol.push_back({ x,p });
	}
}


int main() {
	freopen("mesaj4.in", "r", stdin);
	freopen("mesaj4.out", "w", stdout);

	scanf("%d %d", &n, &m);

	for (int i = 0; i < m; ++i) {
		int x, y;
		scanf("%d %d", &x, &y);
		g[x].push_back(y);
		g[y].push_back(x);
	}

	if (!iSConnected()) {
		printf("-1");
		return 0;
	}

	memset(v, 0, sizeof(v));
	dfsAquire(1, 0);

	printf("%d\n", (n - 1) * 2);
	for (auto x : sol) {
		printf("%d %d\n", x.first, x.second);
	}
	for (int i = sol.size() - 1; i >= 0; i--) {
		printf("%d %d\n", sol[i].second, sol[i].first);
	}

	return 0;
}