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

#define MAXN 100005

int n, m;

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

struct Event {
	int x, y, i;
};

stack<Event> s;

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);
	}

	s.push({ 1, 0, 0});

	while (!s.empty()) {
		auto x = s.top();
		s.pop();
		
		bool didPush = false;
		v[x.x] = true;

		for (int i = x.i; i < g[x.x].size(); ++i) {
			auto y = g[x.x][i];
			if (!v[y]) {
				s.push({ x.x,x.y,i + 1 });
				s.push({ y,x.x,0 });
				didPush = true;
				break;
			}
		}
		if (!didPush) {
			if (x.y != 0) {
				sol.push_back({ x.x,x.y });
			}
		}

	}

	if (sol.size() < n - 1) {
		printf("-1");
		return 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;
}