#include <fstream>
#include <vector>
std::ifstream fin("mesaj4.in");
std::ofstream fout("mesaj4.out");
bool use[100002];
int n,m,nrnod;
using std::vector;
vector<std::pair<int,int>> s;
vector<vector<int>> v;
void dfs(int nod)
{
    use[nod]=1; ++nrnod;
    for(int i=0;i<(int)v[nod].size();++i)
        if(!use[v[nod][i]])
        {
            s.push_back(std::make_pair(nod,v[nod][i]));
            dfs(v[nod][i]);
        }
}
void af(int x)
{
    if(x)
    {
        fout<<s[x-1].second<<" "<<s[x-1].first<<"\n";
        af(x-1);
        fout<<s[x-1].first<<" "<<s[x-1].second<<"\n";
    }
}
int main()
{
    fin>>n>>m;
    int x,y;
    v.resize(n+2);
    for(int i=0; i<m; i++)
    {
        fin>>x>>y;
        v[x].push_back(y);
        v[y].push_back(x);
    }
    dfs(1);
    if(nrnod!=n)
    {
        fout<<-1<<"\n";
        return 0;
    }
    fout<<2*n-2<<"\n";
    af(n-1);
    return 0;
}