#include <bits/stdc++.h>

using namespace std;

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

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

void dfs(int node)
{
    int it;
    viz[node] = 1;
    for(int i = 0 ; i < v[node].size() ; i++)
    {
        it = v[node][i];
        if(!viz[it])
        {
            dfs(it);
            ans[++k] = {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(k != n - 1)
    {
        g << -1;
        return 0;
    }
    g << 2 * (n - 1) << "\n";
    for(i = 1 ; i <= k ; i++)
        g << ans[i].first << " " << ans[i].second << "\n";

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


    return 0;
}