#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;

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

vector <int> G[100001];
int n,m;
bool viz[100001];

struct muchie{int i,j;};
vector <muchie> M;

void dfs(int nod)
{
    viz[nod]=1;
    for(auto vecin:G[nod])
        if(!viz[vecin])
        {
            dfs(vecin);
            M.push_back({vecin,nod});
        }
}

int main()
{
    cin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        int a,b;
        cin>>a>>b;
        if(find(G[a].begin(), G[a].end(), b)==G[a].end())
        {
            G[a].push_back(b);
            G[b].push_back(a);
        }
    }
    for(int i=1;i<=n;i++)
        if(!viz[i])
            if(i==1) dfs(i);
            else
            {
                cout<<-1;
                return 0;
            }
    cout<<2*(n-1)<<endl;

    for(auto e:M)
        cout<<e.i<<" "<<e.j<<'\n';
    for(int i=M.size()-1;i>=0;i--)
        cout<<M[i].j<<" "<<M[i].i<<'\n';
    return 0;
}