#include<cstdio>
#include<vector>
using namespace std;
vector<int>L[100100];
struct muc{
    int a;
    int b;
}x[400100];
int n,m,i,j,a,b,nr,v[100100];
FILE *f,*g;
void dfs(int nod){
    v[nod] = 1;
    for(int i=0; i < L[nod].size(); i++ ){
        if( v[ L[nod][i] ] == 0 ){
            dfs( L[nod][i] );
            nr++;
            x[nr].a = nod;
            x[nr].b = L[nod][i];
        }
    }
}
int main(){
    f=fopen("mesaj4.in","r");
    g=fopen("mesaj4.out","w");
    fscanf(f,"%d%d",&n,&m);
    for(i=1;i<=m;i++){
        fscanf(f,"%d%d",&a,&b);
        L[a].push_back(b);
        L[b].push_back(a);
    }
    dfs(1);
    if( nr != n - 1 ){
        fprintf(g,"-1");
        return 0;
    }
    fprintf(g,"%d\n",2*nr);
    for(i=1;i<=nr;i++){
        fprintf(g,"%d %d\n",x[i].b,x[i].a);
    }
    for(i=nr;i>=1;i--){
        fprintf(g,"%d %d\n",x[i].a,x[i].b);
    }


    fclose(f);
    fclose(g);
    return 0;
}