#include <bits/stdc++.h>

using namespace std;

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

const int N = 1e5;

int n, m;

vector <int> edges[N + 1];

int viz[N + 1];

vector <pair <int, int>> v;

void dfs(int nod)
{
    viz[nod] = 1;
    for(auto x:edges[nod])
    {
        if(!viz[x])
        {
            v.push_back({x, nod});
            viz[x] = 1;
            dfs(x);
        }
    }
}

int main()
{
    in >> n >> m;
    for(int i = 1; i <= m; i++)
    {
        int x, y;
        in >> x >> y;
        edges[x].push_back(y);
        edges[y].push_back(x);
    }
    dfs(1);
    for(int i = 1; i <= n; i++)
    {
        if(!viz[i])
        {
            out << -1;
            return 0;
        }
    }
    out << 2 * (n - 1) << '\n';
    for(int i = v.size() - 1; i >= 0; i--)
        out << v[i].first << ' ' << v[i].second << '\n';
    for(int i = 0; i < v.size(); i++)
        out << v[i].second << ' ' << v[i].first << '\n';
    return 0;
}