#include<fstream>
#include<bitset>
#include<vector>
using namespace std;
ifstream f("mesaj4.in");
ofstream g("mesaj4.out");
bitset<100010>viz;
vector<int>l[100010];
int n,m,nr;
pair<int,int>fin[100010];
void dfs(int x)
{
    viz[x]=1;
    for(int i=0;i<l[x].size();++i)
      if(viz[l[x][i]]==0)
      {
          dfs(l[x][i]);
          ++nr;
          fin[nr]=make_pair(x,l[x][i]);
      }
}
int main()
{
    f>>n>>m;
    for(int i=1;i<=m;++i)
    {
        int x,y;
        f>>x>>y;
        l[x].push_back(y);
        l[y].push_back(x);
    }
    dfs(1);
    if(nr!=n-1)
      g<<-1;
    else
    {
        g<<nr*2<<"\n";
        for(int i=1;i<=nr;++i)
          g<<fin[i].second<<" "<<fin[i].first<<"\n";
        for(int i=nr;i>=1;--i)
          g<<fin[i].first<<" "<<fin[i].second<<"\n";
    }
    return 0;
}