#include <bits/stdc++.h>

using namespace std;

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

const int NMAX = 100005;
int n,m, viz[NMAX];
vector <int> v[NMAX];
vector <pair <int, int > > ans;

void dfs(int node){
    viz[node] = 1;
    for(auto it: v[node])
        if(!viz[it]){
            dfs(it);
            ans.push_back(make_pair(it, 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(ans.size() != n - 1){
        g << -1;
        return 0;
    }
    g << 2 * (n - 1) << "\n";
    for(pair <int, int> it: ans)
        g << it.first << " " << it.second << "\n";

    for(i = ans.size() - 1 ; i >= 0 ; i--)
        g << ans[i].second << " " << ans[i].first << "\n";


    return 0;
}