#include<iostream>
#include<fstream>
#include<vector>
using namespace std;
int viz[100004],sol_a[100004],sol_b[100004],i,n,nr,a,b,m;
vector<int> v[100004];
void dfs(int k)
{
    int i;
    viz[k]=1;
    for(i=0;i<v[k].size();i++)
    {
        if(viz[v[k][i]]==0)
        {
            nr++;
            sol_b[nr]=v[k][i];
            sol_a[nr]=k;
            dfs(v[k][i]);

        }
    }
}
int main()
{
    ifstream cin("mesaj4.in");
    ofstream cout("mesaj4.out");
    cin>>n>>m;
    for(i=1;i<=m;i++)
     {
         cin>>a>>b;
         v[a].push_back(b);
         v[b].push_back(a);
         
     }
     dfs(1);
     if(nr!=n-1)
         cout<<"-1";
     else{
         cout<<nr*2<<"\n";
         for(i=1;i<=nr;i++)
             cout<<sol_a[i]<<" "<<sol_b[i]<<"\n";
        for(i=nr;i>=1;i--)
            cout<<sol_b[i]<<" "<<sol_a[i]<<"\n";
     }
     return 0;
}