#include<fstream>
#include<vector>
using namespace std;
int n, m, nr;
vector<int>tr[100002];
vector<pair<int, int> >ans1;
bool viz[100002];
void dfs(int dad, int nod)
{
    viz[nod] = 1;
    for(int i = 0; i < tr[nod].size(); ++i)
    {
        int vecin = tr[nod][i];
        if(viz[vecin])
            continue;
        dfs(nod, vecin);
        ans1.push_back({vecin, nod});
    }
}
int main()
{
    ifstream f("mesaj4.in");
    f >> n >> m;
    for(int i = 1; i <= m; ++i)
    {
        int a, b;
        f >> a >> b;
        tr[a].push_back(b);
        tr[b].push_back(a);
    }
    dfs(0, 1);
    ofstream g("mesaj4.out");
    if(ans1.size() != n-1)
    {
        g << -1;
        return 0;
    }
    g << 2 * (n-1) << '\n';
    for(int i = 0; i <= n-2; ++i)
        g << ans1[i].first << " " << ans1[i].second << '\n';
    for(int i = n - 2; i >= 0; --i)
        g << ans1[i].second << " " << ans1[i].first << '\n';
    return 0;
}