#include <iostream>
#include <fstream>
#include <vector>
#define N 100005
#define M 500005
using namespace std;
ifstream fin ("ciclueuler.in");
ofstream fout ("ciclueuler.out");
int n,m,viz[M];
vector <pair<int,int> > a[N];
vector <int> q,v;
void dfs(int i)
{
    int u;
    q.push_back(i);
    while (!q.empty())
    {
        u=q.back();
        if (!a[u].empty())
        {
            pair <int , int> w;
            w=a[u].back();
            a[u].pop_back();
            if (!viz[w.second])
            {
                viz[w.second]=1;
                q.push_back(w.first);
            }
        }
        else
        {
            q.pop_back();
            v.push_back(u);
        }
    }
}
int main()
{
    int x,y,i;
    fin>>n>>m;
    for (i=1;i<=m;i++)
    {
        fin>>x>>y;
        a[x].push_back(make_pair(y,i));
        a[y].push_back(make_pair(x,i));
    }
    for (i=1;i<=n;i++)
        if (a[i].size()%2==1)
        {
            fout<<"-1"<<"\n";
            return 0;
        }
    dfs(1);
    fout<<m<<"\n";
   for (i=0; i<v.size()-1; i++)
        fout<<v[i]<<" ";
    fin.close();
    fout.close();
    return 0;
}