#include <cstdio>
#define MAXN 100000
#define MAXM 100000
int ans, u, k, viz[MAXN+1], val[2*MAXM+1], next[2*MAXM+1], lista[MAXN+1], a[2*MAXN], b[2*MAXN];
inline void adauga(int x, int y){
    k++;
    val[k]=y;
    next[k]=lista[x];
    lista[x]=k;
}
void dfs(int x){
    int p=lista[x];
    viz[x]=1;
    u++;
    while(p){
        if(viz[val[p]]==0){
            dfs(val[p]);
            a[ans]=val[p];
            b[ans]=x;
            ans++;
        }
        p=next[p];
    }
}
int main(){
    int n, m, x, y, i;
    FILE *fin, *fout;
    fin=fopen("mesaj4.in", "r");
    fout=fopen("mesaj4.out", "w");
    fscanf(fin, "%d%d", &n, &m);
    for(i=0; i<m; i++){
        fscanf(fin, "%d%d", &x, &y);
        adauga(x, y);
        adauga(y, x);
    }
    dfs(1);
    if(u!=n){
        fprintf(fout, "-1\n");
    }else{
        fprintf(fout, "%d\n", 2*ans);
        for(i=0; i<ans; i++){
            fprintf(fout, "%d %d\n", a[i], b[i]);
        }
        for(i=2*ans-1; i>=ans; i--){
            fprintf(fout, "%d %d\n", b[i-ans], a[i-ans]);
        }
    }
    fclose(fin);
    fclose(fout);
    return 0;
}