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

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

vector<vector<int> > G;
vector<int> viz;
vector<int> drum;
queue<int> Q;
int N, M, X, B, l;

void citire()
{
    in >> N >> M;
    G.resize(N + 2);
    int x, y;
    for(int i = 1; i <= M; i++)
    {
        in >> x >> y;
        G[y].push_back(x);
        G[x].push_back(y);
    }
}

void BFS(int z)
{
    Q.push(z);
    viz[z] = 1;
    int Vecin;
    while(!Q.empty())
    {
        l++;
        int Nod = Q.front();
        for(int i = 0; i < G[Nod].size(); i++)
        {
            Vecin = G[Nod][i];
            if(!viz[Vecin])
            {
                drum.push_back(Nod);
                drum.push_back(Vecin);
                viz[Vecin] = viz[Nod] + 1;
                Q.push(Vecin);
            }
        }
        Q.pop();
    }
}

int main()
{
    citire();
    viz.resize(N + 2);
    BFS(1);
    if(l != N)
    {
        out << -1;
        return 0;
    }
    out << drum.size() << '\n';
    for(int i = 2 * N - 3; i > 0; i -= 2)
        out << drum[i] << ' ' << drum[i - 1] << '\n';
    for(int i = 0; i < 2 * N - 2; i += 2)
        out << drum[i] << ' ' << drum[i + 1] << '\n';
//    for(int i = 1; i <= N; i++)
//        cout << viz[i] << ' ';
}