#include <iostream>
#include <fstream>
#include <vector>
#include <bitset>
using namespace std;

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

vector<vector<int>> G;
vector<pair<int, int >> s;
vector<bool> b;

int n, m;

void Read();
void Dfs(int x);
void Write();

int main()
{
    Read();
    Dfs(1);
    Write();

    fin.close();
    fout.close();

    return 0;
}

void Read()
{

    fin >> n >> m;
    int x, y;
    G = vector<vector<int>>(n + 1, vector<int>());
    b = vector<bool>(n + 1, 0);
    for ( int i = 1; i <= m; i++ )
    {
        fin >> x >> y;
        G[x].push_back(y);
        G[y].push_back(x);
    }
}

void Dfs(int x)
{
    b[x]=1;
    for ( auto y : G[x] )
    {
        if ( !b[y] )
        {

            Dfs(y);
            s.push_back({y, x});
        }
    }
}

void Write()
{
    if ( s.size() != n - 1 )
    {
         fout << -1;
         return;
    }

    fout << 2 * s.size() << "\n";

    for ( auto y : s )
        fout << y.first << ' ' << y.second<< "\n";

    for (vector<pair<int, int>>::iterator it = s.end() - 1; it >= s.begin(); --it )
        fout << it->second << ' ' << it->first << "\n";
}