#include<bits/stdc++.h>
using namespace std;
ifstream f("mesaj4.in");
ofstream g("mesaj4.out");
int n, m, tt[100002], rg[100002], nr;
vector<int>tr[100002];
vector<pair<int, int> >ans1, ans2;
int Find(int a)
{
    if(tt[a] == a)
        return a;
    return tt[a] = Find(tt[a]);
}
void Union(int a, int b)
{
    if(rg[a] >= rg[b])
        tt[b] = a, rg[a] += rg[b];
    else
        tt[a] = b, rg[b] += rg[a];
}
void dfs(int dad, int nod)
{
    for(int i = 0; i < tr[nod].size(); ++i)
    {
        int vecin = tr[nod][i];
        if(vecin == dad)
            continue;
        ans2.push_back({nod, vecin});
        dfs(nod, vecin);
        ans1.push_back({vecin, nod});
    }
}
int main()
{
    f >> n >> m;
    for(int i = 1; i <= n; ++i)
        tt[i] = i, rg[i] = 1;
    for(int i = 1; i <= m; ++i)
    {
        int a, b;
        f >> a >> b;
        if(Find(a) != Find(b))
            Union(Find(a), Find(b)), tr[a].push_back(b), tr[b].push_back(a), ++nr;
    }
    if(nr != n-1)
    {
        g << -1;
        return 0;
    }
    dfs(0, 1);
    g << 2 * (n-1) << '\n';
    for(int i = 0; i < ans1.size(); ++i)
        g << ans1[i].first << " " << ans1[i].second << '\n';
    for(int i = 0; i < ans2.size(); ++i)
        g << ans2[i].first << " " << ans2[i].second << '\n';
    return 0;
}