#include <fstream>
#include <vector>

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

const int Nmax = 100005;
int n,m;
bool Use[Nmax];
vector <int> G[Nmax];
pair <int, int> sol[Nmax];

void Read()
{
    f>>n>>m;
    while(m--)
    {
        int x, y;
        f>>x>>y;
        G[x].push_back(y);
        G[y].push_back(x);
    }
}

void DFS(int nod)
{
    Use[nod] = 1;
    for(int i = 0; i < (int)G[nod].size(); i++)
    {
        int vecin = G[nod][i];
        if(!Use[vecin])
        {
            sol[++m] = make_pair(nod, vecin);
            DFS(vecin);
        }
    }
}

void Print()
{
    if(m != n-1)
    {
        g<<-1<<'\n';
        return;
    }
    g<<2*(n-1)<<'\n';
    for(int i = n-1; i >= 1; i--)
    {
        g<<sol[i].second<<' '<<sol[i].first<<'\n';
    }
    for(int i = 1; i < n; i++)
    {
        g<<sol[i].first<<' '<<sol[i].second<<'\n';
    }
}

int main()
{
    Read();
    m = 0;
    DFS(1);
    Print();
    return 0;
}