#include<fstream>
#include<vector>
#include<cstring>
#include<cstdio>
#define DIM 100005
using namespace std;
int n, m, i, x, y;
vector<int> v[DIM];
int viz[DIM];
FILE * fin = fopen("mesaj4.in", "r");
FILE * fout = fopen("mesaj4.out", "w");
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){
         fprintf(fout, "%d %d\n", t, nod);
    }
}
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){
             fprintf(fout, "%d %d\n", nod, vecin);
            dfs2(vecin);
        }
    }
}
int main(){
    fscanf(fin, "%d%d", &n, &m);
    for(i = 1; i <= m; i++){
        fscanf(fin, "%d%d", &x, &y);
        v[x].push_back(y);
        v[y].push_back(x);
    }
    dfs(1);
    for(i = 1; i <= n; i++){
        if(viz[i] == 0){
            fprintf(fout, "-1\n");
            return 0;
        }
    }
     fprintf(fout, "%d\n", 2 * (n - 1));
    memset(viz, 0, sizeof(viz));
    dfs1(1, 0);
    memset(viz, 0, sizeof(viz));
    dfs2(1);
    return 0;
}