#include<fstream>
#include<vector>

using namespace std;

ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");

vector<vector<int>>v;
vector<bool>ap;
vector<pair<int,int>>pereche;

inline void DFS(int start)
{
    ap[start] = true;
    for (int i = 0; i < v[start].size(); i++)
    {
        if(ap[v[start][i]] == false)
        {
            pereche.push_back({start, v[start][i]});
            DFS(v[start][i]);
        }
    }
}

int main()
{
    int X, U;
    fin >> X >> U;
    v.resize(X);
    ap.resize(X);
    for(int i = 0; i < U ; i++)
    {
        int V1, V2;
        fin >> V1 >> V2;
        V1--, V2--;
        v[V1].push_back(V2);
        v[V2].push_back(V1);
    }
    DFS(false);
    if(pereche.size() == X - 1)
    {
        fout << 2 * X - 2 << '\n';
        for(int i = X - 2; i >= 0; i--)
        {
            fout << pereche[i].second + 1 << ' ' << pereche[i].first + 1 << '\n';
        }
        for(int i = 0; i < X - 1; i++)
        {
            fout << pereche[i].first + 1 <<  " " << pereche[i].second + 1 << '\n';
        }
    }
    else
    {
        fout << -1;
    }
    return false;
}