#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using pi = pair<int, int>;
using ti = tuple<int, int, int>;
using tl = tuple<ll, ll, ll>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pl = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vpi = vector<pi>;
using vpil = vector<pil>;
using vpli = vector<pli>;
using vpl = vector<pl>;
using vti = vector<ti>;
using vtl = vector<tl>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using vvpi = vector<vpi>;
using vvpil = vector<vpil>;
using vvpli = vector<vpli>;
using vvpl = vector<vpl>;
using vvti = vector<vti>;
using vvtl = vector<vtl>;


#if 1
ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");
#define cin fin
#define cout fout
#endif
const int nmx = 1e5 + 1;
vi a[nmx];
vpi paths;
int vis[nmx];
void Dfs(int k)
{
	vis[k] = 1;
	for (int to : a[k])
	{
		if (vis[to] == 0)
		{
			paths.emplace_back(k, to);
			Dfs(to);
		}
	}
}
int main()
{
	int n,m;
	cin >> n >> m;
	for (int i = 0; i < m; i++)
	{
		int x, y;
		cin >> x >> y;
		a[x].emplace_back(y);
		a[y].emplace_back(x);
	}
	Dfs(1);
	for(int i=1;i<=n;i++)
		if (vis[i] == 0)
		{
			cout << -1;
			return 0;
		}
	cout << paths.size() * 2 << "\n";
	for (pi& p : paths)
		cout << p.first << " " << p.second << "\n";
	for (int i = paths.size() - 1; i >= 0; i--)
	{
		cout << paths[i].second << " " << paths[i].first << "\n";
	}
}