#include <fstream>
#include <utility>
#include <vector>

using namespace std;

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

const int NMax = 1e5;

vector <int> G[NMax + 5]; pair <int, int> V[NMax + 5];

int N, M, K; bool Use[NMax + 5];

void DFS(int Nod, int Tata)
{
    Use[Nod] = 1; V[++K] = {Tata, Nod};

    for(auto Vecin : G[Nod])
        if(!Use[Vecin])
            DFS(Vecin, Nod);
}

int main()
{
    fin >> N >> M;

    for(int i = 1, a, b; i <= M; i++)
    {
        fin >> a >> b;

        G[a].push_back(b);
        G[b].push_back(a);
    }
    DFS(1, 0);

    for(int i = 1; i <= N; i++)
        if(!Use[i])
        {
            fout << "-1\n";
            return 0;
        }
    fout << 2 * (N - 1) << '\n';

    for(int i = K; i > 1; i--)
        fout << V[i].second << " " << V[i].first << '\n';

    for(int i = 2; i <= K; i++)
        fout << V[i].first << " " << V[i].second << '\n';

    fin.close();
    fout.close();

    return 0;
}