#include<fstream>
#include<vector>
#include<cstring>
#define DIM 100005
using namespace std;
int n, m, i, x, y;
vector<int> v[DIM];
int viz[DIM];
ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");
void dfs(int nod){
    viz[nod] = 1;
    for(int i = 0; i < v[nod].size(); i++){
        int vecin = v[nod][i];
        if(viz[vecin] == 0){
            dfs(vecin);
        }
    }
}
void dfs1(int nod, int t){
     viz[nod] = 1;
    for(int i = 0; i < v[nod].size(); i++){
        int vecin = v[nod][i];
        if(viz[vecin] == 0){
            dfs1(vecin, nod);
        }
    }
    if(t != 0){
        fout<< nod <<" "<< t <<"\n";
    }
}
void dfs2(int nod){
     viz[nod] = 1;
    for(int i = 0; i < v[nod].size(); i++){
        int vecin = v[nod][i];
        if(viz[vecin] == 0){
            fout<< nod <<" "<< vecin <<"\n";
            dfs2(vecin);
        }
    }
}
int main(){
    fin>> n >> m;
    for(i = 1; i <= m; i++){
        fin>> x >> y;
        v[x].push_back(y);
        v[y].push_back(x);
    }
    dfs(1);
    for(i = 1; i <= n; i++){
        if(viz[i] == 0){
            fout<<"-1\n";
            return 0;
        }
    }
    fout<< 2 * (n - 1) <<"\n";
    memset(viz, 0, sizeof(viz));
    dfs1(1, 0);
    memset(viz, 0, sizeof(viz));
    dfs2(1);
    return 0;
}