#include <fstream>
#include <vector>
#include <cstring>
using namespace std;
ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");
int n,m,viz[100001],nr;
vector <int> A[100001];
void dfs(int nod,int x)
{
    viz[nod]=1;

    for(auto i:A[nod])
    {

        if(viz[i]==0)
        {
            if(x==1)
            fout<<nod<<" "<<i<<"\n";
            dfs(i,x);
            if(x==1)
            fout<<i<<" "<<nod<<"\n";
        }
    }
}
int main()
{
    fin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        int x,y;
        fin>>x>>y;
        A[x].push_back(y);
        A[y].push_back(x);
    }
    for(int i=1;i<=n;i++)
    {
        if(viz[i]==0)
        {
            nr++;
            dfs(i,0);
        }
    }
    if(nr!=1)
    {
        fout<<-1<<"\n";
        return 0;
    }
    memset(viz,0,sizeof(viz));
    fout<<(n-1)*2<<"\n";
    dfs(1,1);

    return 0;
}