#include <fstream>
#include <vector>
#include <cstring>
using namespace std;
ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");
int n,m,viz[100001],nr,k;
vector <int> A[100001];
vector <pair <int,int>> sol;
void dfs(int nod)
{
    viz[nod]=1;

    for(auto i:A[nod])
    {

        if(viz[i]==0)
        {
            k++;
            sol.push_back({i,nod});
            dfs(i);

        }
    }
}
int main()
{
    fin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        int x,y;
        fin>>x>>y;
        A[x].push_back(y);
        A[y].push_back(x);
    }
    for(int i=1;i<=n;i++)
    {
        if(viz[i]==0)
        {
            nr++;
            dfs(i);
        }
    }
    if(nr!=1)
    {
        fout<<-1<<"\n";
        return 0;
    }

    fout<<2*k<<"\n";
    for(int i=k-1;i>=0;i--)
        fout<<sol[i].first<<" "<<sol[i].second<<"\n";
    for(int i=0;i<k;i++)
   fout<<sol[i].second<<" "<<sol[i].first<<"\n";
    return 0;
}