#include <fstream>
#include <vector>
using namespace std;

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

int n, m, t, tx[100001], ty[100001];
bool f[100001];
vector<int> g[100001];

void read()
{
	in >> n >> m;
	int x, y;
	for(int i = 0; i < m; ++i)
		in >> x >> y,
		g[x].push_back(y),
		g[y].push_back(x);
}

void dfs(int s)
{
	f[s] = 1;
	for(auto i : g[s])
		if(!f[i])
			tx[t] = s,
			ty[t++] = i,
			dfs(i);
}

int main()
{
	read();
	dfs(1);
	if(t == n-1)
	{
		out << t*2 << '\n';
		for(int i = t-1; i >= 0; --i)
			out << ty[i] << ' ' << tx[i] << '\n';
		for(int i = 0; i < t; ++i)
			out << tx[i] << ' ' << ty[i] << '\n';
	}
	else out << -1;
	return 0;
}