#include <iostream>
#include <fstream>
#include <cstring>
#include <vector>
#include <bitset>
using namespace std;
ifstream in("mesaj4.in");
ofstream out("mesaj4.out");
const int maxn = 100005;
pair <int, int> rec[maxn];
vector <int> g[maxn];
bitset <maxn> viz;
int cnt;
int m;
void dfs(int x)
{
    viz[x] = 1;
    for(auto it : g[x])
    {
        if(!viz[it])
        {
            dfs(it);
            cnt++;
            rec[cnt] = make_pair(x, it);
        }
    }
}

void citire()
{
    string s = "";
    getline(in, s);
    for(int i = 1; i <= m; i++)
    {
        getline(in, s);
        int p = s.size();
        int poz = 0;
        int x = 0;
        while(s[poz] >= '0' && s[poz] <= '9')
        {
            x = x * 10 + s[poz] - '0';
            poz++;
        }
        poz++;
        int y = 0;
        for(int j = poz; j < p; j++)
            y = y * 10 + s[j] - '0';
        g[x].push_back(y);
        g[y].push_back(x);
    }
}

int main()
{
    int n;
    in >> n >> m;
    citire();
    dfs(1);
    if(cnt < n - 1)
    {
        out << -1 << "\n";
        return 0;
    }
    out << cnt * 2 << "\n";
    for(int i = 1; i <= cnt; i++)
        out << rec[i].second << " " << rec[i].first << "\n";
    for(int i = cnt; i >= 1; i--)
        out << rec[i].first << " " << rec[i].second << "\n";
    return 0;
}