#include <bits/stdc++.h>

using namespace std;

ifstream f("mesaj4.in");
ofstream g("mesaj4.out");

const int NMAX = 100005;
int n,m;
bitset <NMAX> viz;
vector <int> v[NMAX];
int a[NMAX], b[NMAX], k;

void dfs(int node){
    viz[node] = 1;
    for(auto it: v[node])
        if(!viz[it]){
            dfs(it);
            a[++k] = it;
            b[k] = node;
        }
}

int main(){
    int i,x,y;
    f >> n >> m;
    for(i = 1 ; i <= m ; i++){
        f >> x >> y;
        v[x].push_back(y);
        v[y].push_back(x);
    }
    dfs(1);

    if(k != n - 1){
        g << -1;
        return 0;
    }
    g << 2 * (n - 1) << "\n";

    for(i = 1 ; i <= k ; i++)
        g << a[i] << " " << b[i] << "\n";
    for(i = k ; i >= 1 ; i--)
        g << b[i] << " " << a[i] << "\n";

    return 0;
}