#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

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

vector <int> g[100005];
vector <pair <int, int>> v;
bool viz[100005];

void dfs(int nod)
{
    viz[nod] = 1;
    for (int i = 0; i < g[nod].size(); i++)
        if (!viz[g[nod][i]])
        {
            v.push_back(make_pair(nod, g[nod][i]));
            dfs(g[nod][i]);
        }
}

int main()
{
    int n, m;
    fin >> n >> m;
    for (int i = 1; i <= m; i++)
    {
        int x, y;
        fin >> x >> y;
        g[x].push_back(y);
        g[y].push_back(x);
    }
    dfs(1);
    bool k = 0;
    for (int i = 1; i <= n; i++)
        if (!viz[i])
            k = 1;
    if (k == 1)
        fout << -1 << '\n';
    else
    {
        fout << 2 * (n - 1) << '\n';
        for (int i = n - 2; i >= 0; i--)
            fout << v[i].second << ' ' << v[i].first << '\n';
        for (int i = 0; i <= n - 2; i++)
            fout << v[i].first << ' ' << v[i].second << '\n';
    }
    return 0;
}