#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;
string s;

void dfs(int node){
    viz[node] = 1;
    for(auto it: v[node])
        if(!viz[it]){
            dfs(it);
            ans.push_back(make_pair(it, node));
        }
}

void read(int &x, int &y){
    getline(f,s);
    int i,j;
    x = y = 0;
    for(i = 0 ; i < s.size() ; i++)
        if(s[i] >= '0' && s[i] <= '9'){
            j = i;
            while(s[i] >= '0' && s[i] <= '9')
                i++;
            i--;

            if(!x)
                for(j ; j <= i ; j++)
                    x = x * 10 + s[j] - '0';
            else
                for(j ; j <= i ; j++)
                    y = y * 10 + s[j] - '0';
        }
}

int main(){
    int i,x,y;
    read(n,m);
    for(i = 1 ; i <= m ; i++){
        read(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";

    reverse(ans.begin(), ans.end());

    for(pair <int, int> it: ans)
        g << it.second << " " << it.first << "\n";


    return 0;
}