#include <bits/stdc++.h>
#define DIM 100005

using namespace std;

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

int N,M,x,y;
vector <int> v[DIM];
vector <pair <int,int> > Sol;
bitset <DIM> viz;

void dfs(int x){

    viz[x]=1;

    for(int i=0;i<v[x].size();i++){
            int vecin = v[x][i];
            if(!viz[vecin]){
                dfs(vecin);
                Sol.push_back(make_pair(x,vecin));
            }
        }

}

int main(){

    fin >> N >> M;

    while(M--){
        fin >> x >> y;
        v[x].push_back(y);
        v[y].push_back(x);
    }

    dfs(1);

    if(Sol.size()!=N-1){
        fout << "-1\n";
        return 0;
    }

    fout << 2 * (N-1) << "\n";

    for(int i=0;i<Sol.size();i++)
        fout << Sol[i].second << " " << Sol[i].first << "\n";

    for(std::vector <pair <int,int> >::reverse_iterator it=Sol.rbegin();it!=Sol.rend();it++)
        fout << it->first << " " << it->second << "\n";


}