#include <fstream>
#include <utility>
#include <vector>
using namespace std;
ifstream in("mesaj4.in");
ofstream out("mesaj4.out");
const int NMax = 100000;
vector <int> G[NMax + 5];pair <int, int> V[NMax + 5];
int N, M, K;bool Use[NMax + 5];
void DFS(int Nod)
{
    Use[Nod] = 1;

    for(auto Vecin : G[Nod])
        if(!Use[Vecin])
            V[++K] = {Nod,Vecin},DFS(Vecin);
}
int main()
{
    in >> N >> M;
    for(int i = 0, x, y; i < M; i++)
    {
        in >> x >> y;
        G[x].push_back(y);
        G[y].push_back(x);
    }
    DFS(1);
    for(int i = 1; i <= N; i++)
        if(!Use[i])
        {
            out << "-1\n";
            return 0;
        }
    out << 2 * (N - 1) << '\n';

    for(int i = K; i >= 1; i--)
        out << V[i].second << " " << V[i].first << '\n';
    for(int i = 1; i <= K; i++)
        out << V[i].first << " " << V[i].second << '\n';
    return 0;
}