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

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

vector<vector<int> > G;
vector<bool> viz;
vector<int> drum;
int N, M, X, B;

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 DFS(int z, int &l)
{
    viz[z] = 1;
    //cout << z << ' ';
    for(int i = 0; i < G[z].size(); i++)
        if(!viz[G[z][i]])
        {
            l++;
            drum.push_back(z);
            drum.push_back(G[z][i]);
            DFS(G[z][i], l);
        }
}


int main()
{
    int l = 0, l2 = 0;
    citire();
    viz.resize(N + 2);
    DFS(1, l);
    if(l != N - 1)
    {
        out << -1;
        return 0;
    }
    viz.clear();
    viz.resize(N + 2);
    DFS(*(drum.end() - 1), l2);
    l *= 2;
    out << l << '\n';
    for(int i = 0; i < 2 * l; i += 2)
        out << drum[i] << ' ' << drum[i + 1] << '\n';
}