#include <fstream>
#include <vector>

using namespace std;

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

const int Nmax = 100000;

struct edge{
    int x,y;
};
edge muchie;
vector <edge> sol;
vector <int> v[Nmax + 5];
int viz[Nmax + 5], n, m, nr;

void read()
{
    f >> n >> m;
    for(int i = 1, a, b; i <= m; i++)
    {
        f >> a >> b;
        v[a].push_back(b);
        v[b].push_back(a);
    }
}

void dfs(int x)
{
    viz[x] = 1;
    for (auto y:v[x])
    {
        if(viz[y] == 1) continue;

        dfs(y);
        ++nr;
        muchie.x = y;
        muchie.y = x;
        sol.push_back(muchie);
    }
}

int main()
{

    read();
    dfs(1);
    if(nr != (n-1))
    {
        g << -1;
        return 0;
    }
    g << 2*(n-1) << "\n";
    for (int i = 0; i < sol.size(); i++)
        g << sol[i].x << " " << sol[i].y << '\n';
    for (int i = sol.size() - 1; i >= 0; i--)
        g << sol[i].y << " " << sol[i].x << '\n';
    return 0;
}