#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
#include <vector>

#define BUF_SIZE 16384
char buf[BUF_SIZE];
int pbuf=BUF_SIZE;
FILE*fi;
inline char nextch(){
    if(pbuf==BUF_SIZE){
        fread(buf, BUF_SIZE, 1, fi);
        pbuf=0;
    }
    return buf[pbuf++];
}
inline int nextnum(){
    int a=0;
    char c=nextch();
    while(c<'0' || c>'9')
        c=nextch();
    while('0'<=c && c<='9'){
        a=a*10+c-'0';
        c=nextch();
    }
    return a;
}

std::vector <int> G[100001];
int vst[100001];
int list[300000][2], last;
int vis;

void dfs(int nod){
    vst[nod]=1;
    vis++;
    for(int i=0;i<G[nod].size();i++)
        if(vst[G[nod][i]]==0){
            dfs(G[nod][i]);
            list[last][0]=G[nod][i];
            list[last][1]=nod;
            last++;
        }
}

int main(){
    int n, m;
    FILE*fo;
    fi=fopen("mesaj4.in","r");
    fo=fopen("mesaj4.out","w");
    n=nextnum();
    m=nextnum();
    for(int i=0;i<m;i++){
        int a=nextnum(), b=nextnum();
        G[a].push_back(b);
        G[b].push_back(a);
    }
    dfs(1);
    if(vis!=n){
        fprintf(fo,"-1");
        return 0;
    }
    fprintf(fo,"%d\n", 2*n-2);
    for(int i=0;i<last;i++)
        fprintf(fo,"%d %d\n", list[i][0], list[i][1]);
    for(int i=last-1;i>=0;i--)
        fprintf(fo,"%d %d\n", list[i][1], list[i][0]);
    fclose(fi);
    fclose(fo);
    return 0;
}