#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(auto z:v[nod])
        if(!use[z])
        {
            dfs(z);
            fout<<z<<" "<<nod<<"\n";
            s.push_back(std::make_pair(nod,z));
        }
}
int main()
{
    fin>>n>>m;
    fout<<2*n-2<<"\n";
    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.close();
        std::ofstream fout("mesaj4.out");
        fout<<-1<<"\n";
        return 0;
    }
    for(int i=n-2;i>=0;i--)fout<<s[i].first<<" "<<s[i].second<<"\n";
    return 0;
}